WD sync completes, but not all files are sync'd

I am running firmware 2.10.310 on a My Cloud Mirror 4TB. I am attempting to use WD Sync to synchronize my Calibre ebook library between two Macs and a PC.

All of the ebook files are on Mac1 and WD Sync status is ‘Up to date’. On Mac 2 it also has the exact same status.
However, on Mac 1 there are 5363 ebooks, wi the newest one added today (10/29). On Mac 2. there are only 5338 ebooks and the latest book was added on 10/26.

Both Macs are on the same LAN as the My Cloud Mirror. Both Macs are using only ethernet, no wifi.

The PC is still syncing, which I expected as it is wifi and as a result things take much longer.

In addition to missing files in the sync process, the My Cloud Mirror is running extremely slowly. This started only after I began using WDSync.

WDsync does not seem ready for prime time.

What’s the original location of the missing files? Are they added to your unit and the other computer if they are copied to a different location?

The source of the files was Mac1, in the directory that is sync’d.

I should add that the directory “target” on the My Cloud NAS was correct, it had all the files, it was Mac 2, that was missing files. Both system’s WD status was ‘up to date’. I did wait about an hour for Mac 2 to catch up, but it never did.