EXS / Recycle - problems finding files (Logic Pro 10.4x)

I’m not sure when this problem started, but I can’t work it out!

I still use recycle (import recycle to EXS) quite a lot. I would use it during a track, then (when finished) move the completed track to a backup hard drive.

When I went on to start a new project, all the EXS instruments I made previously with recycle import still appeared in the list of sampler instruments - and were able to find the associated audio files for the EXS instruments without any problems.

I’ve recently noticed that when I try to load some of the newer EXS instruments made with recycle convert (newer as in, ones I made in the last year or so, maybe longer) - EXS is unable to find the associated audio files. EXS is set to look for samples on all drives.

Even more strange: if I go to my backup drive and (as a test) load up a old song which I know had recycled EXS instruments it, EXS still can’t find the files - even though I can see the files it seeks are in the Media folder of the same Logic Package. And if I click on the ‘edit’ button of one of the problem EXS instruments, whilst I can see there is the ! symbol next to each ‘missing’ sample, if I click on the name of the missing sample to the left of the !, it brings up the 'missing' waveform and I'm able to play it ok from that window.

Any suggestions welcome. Thanks.