Cannot move project

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I accidentally created a sub-folder and was storing designs in it (drawings, for what they're worth, and models). I decided to have a bit of a tidy up and wanted to move some of these projects to the folder they should be in. I moved most of them, however some can't be moved, and I get the spinning wheel of death on the file. The destination folders in the Move To pop-up are also greyed out.
Some of these projects I've been working on recently, others haven't been opened for a while. It doesn't seem to make any difference there. Occasionally I can't delete models as it says the model is in use, but if I open that project and close it again I can delete it. In this case I've tried to open the projects, even make a slight change, and close them again and it doesn't make any difference either. Absolutely does not want to even think about moving them.
Not sure what to try next?