I solved this issue but thought it might help out someone else struggling with the same problem.
In the experiment, participants view videos, then use the slider to rate them on different dimensions. The experiment ran perfectly in PsychoPy3, but while piloting on Pavlovia sometimes the videos would not play - rather, there was a blank screen for the duration of the movie, and then it would continue on to the next routine as usual. The videos were all .mp4’s, and the issue was highly variable - it wasn’t the same videos every time, and sometimes it played totally fine.
My best guess is that the movie sometimes wasn’t completely loaded before the routine started. Adding a 2000ms delay prior to each movie (changing the start time to 2 seconds) seemed to resolve the issue.