Why are my mp4 files greyed out?

Hi, I recently recorded a few short MP4 video clips on my Samsung Galaxy mobile phone in order to transfer them to my iMac and then transfer them along with some photographs to Davinci Resolve to create a YouTube vlog, the problem that I have with some (but not all) of the MP4 files is they are greyed out on my desktop after transferring them from my phone via Android Transfer, they play perfectly well on my phone so I know the files are there but in the thumbnail on my desktop I can see that there is a grey diagonal line across the file and when I open it and play it 75% of the screen is blacked out. Can anyone suggest a way of fixing this? is it a Mac/Android compatibility issue or something more sinister? I've been doing this for a while now and as far as I can remember it has only started happening recently. The MP4 clips make up an important part of what will be the final edit of the vlog, I would really appreciate any help in fixing the issue if it's possible. Thanks in advance.