Priortizing between different projects and employers
What I'm reading in your description is that you're seeing the effect of your employees prioritizing other projects than your own. I've personally seen similar situations from various perspectives in e.g. semi-outsourced academic research projects.
"a contract ranging from 15-25 hours/mo - they usually have a fulltime job in addition to this project." means that there's some inherent competition regarding time; the requirements of their fulltime job will vary, their needs of family time will vary, and apparently when they need/want to work less hours and something needs to be cut, then they're repeatedly choosing to cut your project.
This doesn't seem surprising. Other things being equal, it makes sense for someone to prioritize their "main" full-time job over a secondary contract, and it makes sense that for someone who has more than enough demand of paid work to do, that they might also choose to prioritize their non-work life above work projects; because if they have too many contract-customers, then possibly losing one contract is not only a big deal but something that's good for them, e.g. if they really should have said "no, I don't really have time for this" but failed to understand and admit this in time.
What can you do?
In essence, you want to "sell" them your tasks as more attractive than the competitor's (the full time job and/or nonwork life), so that they start choosing to spend more hours working gor you. Given the description, it seems that the threat of firing them isn't an effective option, so there are essentially two main paths here.
Become their main job
One is to ensure that they treat your project as their main job. Not necessarily as the only job (there may be all kinds of situations where that wouldn't be possible), but so that they emotionally feel that this is their most important job and, when push comes to shove, it'll be the other one where they'll fail the deadlines (and possibly risk losing it) in order to spend enough effort on your project.
This generally will require you to spend more money, so that you also become their main source of income - either the "true contractor strategy", in which the contract would have multiple times higher hourly rate so a 25 hours/mo contract would be a significant part of their total compensation, comparable to whatever their full-time job pays, making it reasonable for them to take time off at that job to focus on the obviously much more lucrative contract; or the "employee" strategy where you book enough of their time so that you're filling most of their time and income, and also providing guarantees of stable income (e.g. in absence of contract customers) so they can treat you as their full-time job, instead of just a contract that comes and goes. I've seen both such options applied in practice, but it's an either-or case, there's no economically reasonable middle road.
Or accept that you're not their main job
The other path is to accept that you won't be their main job ever, and you will be essentially buying small amounts their spare time up to the extent that they're deciding to sell it this week, and get better at "herding cats". All kinds of nonfinancial motivation, weird carrot and stick approaches, possibly even gamification might be very helpful. The weekly reviews you describe are a good option in my experience, however, the personal meetings might need to be more frequent. It may be profitable to spend a lot more hands-on management time to understand what exactly motivates them (because apparently this isn't their carreer, their main income comes from elsewhere, this is just a side gig. The money needs to be respectable as a 'hygiene factor', but they're likely not doing it just or mainly for money), and rearrange the world so that the things that they desire (and don't get in their full-time job) happen to happen in this project iff they're productive enough and achieve your goals instead of that other employer's goals. If they emotionally consider your project as a somewhat compensated hobby, then it can work as well and get a lot of good, highly motivated production as long as it feels pleasant to them.
In this path, what I've seen in practice is that the problem tends to be not with the quantity or quality of the work, but with the reliability of it. Vague goals with no deadlines (or deadlines that are understood to be arbitrary) are a trap for procastination, and doing other tasks than your project (e.g. the "urgent but not important" things from Eisenhower's matrix, they probably have more than enough of them in their full time job). Reasonable ways to increase reliability of doing task X before the deadline that you want are (a) demonstrate the true importance of these deadlines - e.g. involve them more with end-users of these tasks; as long as there's some personal connection and they see how/why these people need that thing yesterday, they'll be more committed to quicker delivery; (b) explicitly assign more meaning to these deadlines (even if arbitrary) e.g. some bonus/penalty that's tied to particular delivery dates, so they'll shift things around in their full-time job to make time for this task; or (c) if the task allows, involving them more in the planning and scheduling of these deadlines and deliverables; giving them more autonomy and responsibility for deciding how/when they're going to do it generally does get people more committed to doing them at the time they decided. But it's still not a guarantee, padding, careful management and motivation still have a big impact.
The 'herding cats' analogy is very relevant once the second half of the standard "we need your work, you need our salary/contract/sales" deal ceases to apply. Perhaps there's some literature on managing volunteer work? This might apply even to cases like this where a reasonably hourly rate is paid, as long as that income isn't material to their lifestyle. Perhaps there actually isn't a problem, and the results you get for the price you pay (including the occasional unreliability) is close to the best one can get in these circumstances? In that case, you "simply" need to plan some slack in the schedule, so that you can deliver on your commitments even if the [sub-]contractors are slightly late on their commitments.
