Moving components into another component is not working

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Hey all, I've searched the forum and found a few answers to this problem but none of them fix what is going on with my design.
I'm deep into an assembly and I've decided that I want to move some components into a sub-assembly, but I can't get them to move at all. When I drag them in the browser, nothing happens. No grey line, no highlight, nothing.
I am at the end of the timeline (one answer I saw)
All the sketches used for each component are a part of that component.
One other post I saw had a small bit about somehow breaking references being an issue, but I don't understand how this would be an issue at the end of the timeline. Also, if it is something like this, how can I determine what the issue is so I can fix it? The behavior of the browser simply not doing anything makes it difficult to diagnose.
Thanks for any info