Late last year, we had a user whose account was missing all email between March 2015 and December 2015 when in caching mode.
The email was present while the user was in on-line mode.
The users computer is running Windows 7 and the 2014SP2 client.

We tried:
Tools|retrieve entire mailbox.
Placing the client in remote mode, which allows you to set the retrieval date range. I've used this in the past for users whose cache didn't go back all the way to their earliest email.
Renaming the cache folder and forcing a recreation of the folder.
We then tried the above with two other computers.
One ran the version 8 client on an XP machine.
The other ran the version 2012 client on Windows 7, then the 2014Sp2 client.

Nothing changed.

I submitted an SR (10981513931) and was told to run a GWCheck stand-alone contents check only against the userxxx.db file, then archiving some of the germane email while in live mode, synching the cache, un-archiving them and syncing the cache again.
The user was in a time-crunch at that point, so we agreed to wait until later to do so.

I now have two more users exhibiting the same problem.
In the second case, all email from 4 February 2015 and 11 June 2015 are missing.
The user is running Windows 10 and the 2014SP2 client.
I've not gotten details on the third occurrence.

The differing time periods would appear to eliminate the possibility of a user-specific issue.

The system is running 2014Sp1Hp1 on OES11SP2.
Not that I think it would matter, but the GroupWise server is running all EXT3; no NSS.

I've asked the second user to try the archive/sync/unarchive/sync process to see if it's successful, but that's not a real solution.

I'll be submitting another SR, but I'd like to know if anyone here in the forums has experienced this or if anyone has an idea regarding possible causes/solutions.