If it's important enough that it's actually blocking team progress then that's a management problem, but nothing in this seems to indicate it's anything like that.
> You see them again a bit later, and they insist they're
> "working on it". Time passes. Again, you hear from them:
> "top of my list".All of these seem more charitable, and contribute less to a toxic work environment, than automatically assuming that all of your colleagues are manipulative, dense or flaky because they have more important things to do.
Just be honest and explain you've been caught up with higher priority issues and haven't made progress. That gives the asker an opportunity to explain why the task is more important than you thought, find someone else with less on their plate to do it, or at the very least understand what's taking so long.