Lightroom images missing after using Photo mechanic

Hi hope someone can help. We have wedding photographers based in diferent locations so our workflow is key to the smooth running. Our workflow is as follows.

1. A photographer shhots a wedding
2. They send the RAW files to the office via Dropbox
3. We import them to a master catalogue in Lightroom
4. We then export a catalogue file along with smart previews and send them back to the photographer to edit. This way once we have the RAW files we have relatively small files to pass back and forth.
5. The photographer edits from the exported catalogue and then just send the edited catalogue file back
6. We then 'Import from an other catalogue'
7. All the ratings, edits, colour corrections, crops etc then attach themselves to the master file in the office
8. Bingo, we can now export the final files for the client.

This system has worked fantastic for years and saves lots of time in copping large files. However, on of my photographers has started using 'Photo Mechanic' and since then when we get the edited catalogue back and import to the master catalogue it does not link to the master RAW files. If I try to locate them it says 'The file is associated with another photo in the catalogue. Each file can only be associated with one photo'

Not sure if Photo Mechanic is changing somthing, the files names are identical but maybe the meta is changing and in turn confusing Lightroom.

Any thoughts on this issue would be great.

Regards

Paul Walker