Why are tasks being delegated via the CEO's assistant rather than employees' supervisors?
Assigning work and then managing the performance of that work is (usually) what the supervisor is supposed to do. If that is the expectation in your workplace, a CEO bypassing the chain of command to assign tasks directly is dysfunctional (though fairly common).
It may be that this is the reason things are not getting done: these ad hoc requests from outside the normal path are disruptive to employees, and interfering with accomplishing their primary tasks as assigned by their supervisors.
I suggest talking to the supervisors...but not primarily about the problem of employees not doing the tasks.
Instead, talk to them about how these assignments from the CEO are perceived and whether they interfere with other work. If there is a problem, work on a plan on how these can get integrated into the normal workflow in the future.
It's quite likely that you should be bringing these tasks to the supervisor of the relevant employee, rather than to the employee directly. If so, you can communicate to the supervisor why this is important, and also you can get some feedback if adding a task creates a capacity problem and endangers other work.
One question would be whether to bring this up to the CEO as well. Given your description of the CEO as hands-off, it is probably just something you can take initiative to do. But it may be important for either you or the supervisors to discuss it with the CEO (especially if the ad hoc requests are making it difficult to manage the work).