This can also happen a lot when you are using multiple GoPro's. You have to rename your files before editing, because many will have the same filenames. I ran into this on a location VNR, where half our shots mysteriously disappeared from the timeline because the clips we wanted had been overwritten by same-named files.
Also check your timecodes in the edit program vs Resolve. I've seen issues with 5d where Avid for example calls it 2 hour timecode, but in resolve it is reading as 26 hour timecode....or vice versa. Something to do with how the timecode is recorded on the 5d, and then interpreted in the programs. PITA
You may find it easier to export a flattened file from your editing program and then just color-correct the flattened file within Resolve. These toy cameras without time-of-day timecode and the ability to name files logically are a huge pain for post. The absence of reel names (or numbers) is also problematic.
If you stay on top of this from day one and use transcodes for everything, it can work -- provided the files are named and given timecode that will avoid conflicts in the final edit lists.