Mouse moves issue

Using the Skim Movie example composition, I basically used the exact same setup but I can't seem to get the mouse moves to do the skimming, extremely intermittent. My movie is ProRes 4444, 560x560 px, ~4 mins in length. 4444 is heavy, but 560x560 should keep it manageable, no? I guess I should try another codec with alpha channel. At a loss, seems like it should work.... Some luck after using a 3 sec test movie to try. What gives? I'd post the movie but it's 2GB, and the comp is the example comp.