Can't move some .pxm files

[st3f]

I'm doing some housekeeping archiving off old .pxd and .pxm files to other directories.
I'm moving them between two Finder windows from iCloud Drive/Pixelmator to iCloud Drive/Archive/yyyy/SomeProjectName/Pixelmator where yyyy is the year I created the file and SomeProjectName is... er... some project name.
The .pxd files are moving fine, but some of the .pxm files refuse to budge (it might just be that these are the oldest), showing a 'can't do this symbol' ( in white) by the mouse pointer.
Neither Pixelmator nor Pixelmator Pro are open when moving the files.
I have a workaround: Copy the files then delete the original (Finder *will* let me delete them). I'd rather fix the root cause if I can, though.
Any idea what's going on and if there's anything I can do to release the files?

[st3f]

Never mind. I think I have it.
One had a synchronisation issue with Pixelmator on my iPad.
Three were aliases.
One I moved with the workaround above (just to test it).
The remaining two were rubbish and got deleted.
Everything else I have tried to move, moved just fine.