How do I force LR Classic to sync photos on external drive?

[Bucky28440143m6pt]

How can I force LR Classic to sync a bunch of photos from my external drive?

I use both LR Classic (12.2) and LR CC depending on workflow across a laptop/iPad, so sync is very important to me. However LR Classic will only sync 153 of 3337 photos in my library. These photos are stored on an external drive, and when the drive is connected I can access and edit all my photos with no issues in LR CC. However since they don't sync, I can't edit the same photos (except the 153 which did sync) in LR CC/LR iPad. LR Classic shows a sync completed green tick.

[JohanElzenga]

Lightroom Classic syncs only those images that are added to a synced collection. If you add an image to a synced collection then that image will sync, regardless of the folder location of that image.

[SeanMcCormack]

It takes a finite time to sync. You need to leave the drive connected. 

On trick that can help force sync is to make a minor change to all the photos. e.g. add +1 contrast or texture or a setting that's unlikely to make a big difference. You'll need to leave it all running until the sync completes.