Builds stuck in "scheduled"

I use a cron job to kick off production tests every 10 minutes. Occasionally, one of these test will get stuck in the “scheduled” or “queued” state for hours. This morning, a build had been waiting for well over 10 hours. Now, it seems that even though I cancelled all incomplete builds, new builds are still getting stuck in scheduled.

Looking through the forum, it appears that has been reported before and fixed in a one-off fashion, but there don’t appear to be any current open reports of the problem.

I’m using circle ci 2.0. The project in question is I briefly experimented with workflows yesterday, but they didn’t seem to solve the problem I’m working on right now.