dropped frames

I import 3840x2160 50p footage from my Canon XF405 camera which is recorded in ProRes 422HQ on a Blackmagic recorder/monitor. Some footage is OK,  but with some footage you get about 5 frames OK, then the last frame is duplicated, then it jumps forward 2 frames, in other words every now and then a frame is missing and is replaced by the previous frame duplicated. What is happening?