Recently, I and 4 other coworkers were working together on an internal initiative. Due to the nature of the project (internal rather than client-facing, etc), it was a rather "informal" project that wasn't assigned from my boss (but we had his approval to work on it whenever we all had available time). This also meant there was no project manager.
As one of the original creators for this initiative, I led a "divide & conquer" meeting with my 4 coworkers, where we created a to-do list and everyone picked which areas they wanted to work on. Once every task was assigned, we scheduled a follow up meeting for a week later, to touch base on progress. I made it clear that we all should communicate to each other in case of blockers, as it was likely we'd be able to help "unblock" each others' issues.
At our follow up meeting, everyone had some updates, except for 1 person who said he didn't have time to work on it. It was surprising since I knew his schedule wasn't full, but I let it slide, assuming it was a one-time thing.
However, this happened again & again - he consistently had very little or no progress, and wasn't communicating that to everyone else. I know that this coworker does, in fact, believe in the goal for what we're trying to accomplish. It seems he really just doesn't manage his time well, and possibly needs a more "firm" approach to hold him accountable, rather than the loose "lets check in every so often" method which I had been using.
Thinking over this in retrospect, I'm wondering what I could've done differently. What are some practical ways that I could have held this coworker more accountable to his assigned tasks, without being overbearing or micromanaging?