Check the NAS to see if it is going to sleep.
Try doing smaller imports.
I have checked that - it isn't.
And I have tried with smaller imports and still get the problem on a random basis.
I think it is an issue with the import in Lightroom:
By way of example - I have a folder with multiple subfolders and around 7k images.
The screenshot shows the Lightroom data for the folder after import and for Adobe Bridge.
The Adobe Bridge data matches the output of a shell script query to find the numbers of each type of files.
Lightroom thinks there are 1035 RAW files and 4369 JPEGs - adding the 1483 shown in the Synchronise dialogue above would give 5852, which is close to a correct count. However, if I click Synchronise, Lightroom shows a wait cursor for a while and then shows 0 files to import.
"Ignore probable duplicates" in import is disabled and "treat JPEGs next to Raw Files as separate photos" is ticked in Lightroom preferences.
(I get the same result if I try an Import/Add as I do if I try the synchronise.)
I get this on both Windows 10 PC and on Mac, although the import numbers on the PC differ slightly from the Mac, (PC finds 5508 images on the same folder structure, but also shows 1483 to be imported to synchronise).
I get this result on a range of folder structures on the Synology,
I suppose I could try dragging the structure from Bridge to Lightroom and see what that produces, but I am not sure if that's a good thing to do or not.
Ultimately, I would prefer it if I could just do an import from Lightroom and get the correct results.
Report as a bug on http://feedback.photoshop.com You have a much bigger chance an Adobe engineer sees it there and will investigate. If your NAS is not misbehaving, It is likely some strange interaction between Lightroom and the NAS. Lightroom has always been flaky in reporting how many new images there are in a folder but this is pretty extreme and the fact that it happens on both Windows 10 and Mac OS X is suspect.
Thanks - I'll do that.
I tried selecting images, dragging and dropping from Bridge on the Lightroom import dialog and provided you do it a folder at a time, selecting images, Lightroom gets all the images. If you drag and drop a folder it goes back to the Lightroom image retrieval behaviour with stalling and missed images.