Video doesn't show up in Indesign

I can't place videos into Indesign. I have done it before but now when I bring it in it's empty and even after opening the media panel it doesn't work. There is no preview or play button.

It is an MP4 file and I've tried lots of different mp4 videos and none work, even ones that have worked before.

Could it have anything to do with the size of the document (80 pages). I can't think what it might be or how to solve it.

Thanks.