It can be a number of problems. The last show I started on turned out to have a bunch of AVC Intra stuff, which Resolve didn’t like. So I went to find the files in Finder, converted them to Prores, and imported the files.
Some times I can’t find the issue at all, so I “bake” the clip. In FCP I set I/O, export. Then import and replace in Resolve.
The first and only time I tried using multi clips in Resolve was a disaster. So I baked it, and manually sliced the thing.
What exactly do you mean by I have tried collapsing all the multiclips? What procedure did you do?