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Movies only show the last frame in online study

URL of experiment: Thttps://pavlovia.org/emmafang/the-perception-of-dynamic-expressions-14

Description of the problem: I have 32 movies and each movie lasts 2 seconds. In each trial, a movie was shown for 2 seconds and disappeared afterwards, and participants were asked to rate on one of the two questions. I checked each movie individually (assign them as the first four trials), all of them can play normally. However, when trials increase, some of them were abnormal (only showing the last frame of the movie). For example, when I finish 14 trials, I will probably see a picture (the last frame of a movie) instead of a movie in 15th trial. It seems that the more trials you run (I have 64 trials in total), the more likely that you will see a picture instead of a movie.
I thought adding ISI may help, but it didn’t. I’m thinking adding some clearing cache during the whole experiment may help. But how? Any thoughts? ALL SUGGESTIONS are VERY MUCH appreciated.