I click on a collection to synchronize that collection with Lightroom.
Most of the times it synchronizes correctly, but now and then the little cloud on the right top of the screen says that it is still synchronizing one or two last photo's. I can wait for hours, but the little cloud keeps on saying the same message (blue circle turning around and around). When I look into Lightroom, those same photo's are synchronized fine, but in Lightroom Classic still the little cloud says it's not. The only way to correct this is to look into the settings in LR Classic. There you can find under the synchronize tab which photo's are still hanging according to LR Classic. Then in Lightroom Classic I take them out of the catalog (via the windows 10 explorer). After re-entering them into the catalog (map and collection), the problem is solved.
The same situation when editting a photo in a map that is also in a collection synchronized to Lightroom: when I'm ready with editting, most of the times the little cloud in the top right of the screen says that synchronizing to LR is OK. But some times it keeps hanging: one or two files that I have editted are still to be synchronize to LR. Looking into LR there they are: editted! But still the little cloud in LR Classic keeps on turning blue (synchronizing)
In both cases when I close LR Classic I get the message that LR Classic is still synchronizing and that next time when I start up again the synchronize process will be reactivated. But you can wait untill .... the little cloud keeps on saying that it's still synchronizing. The solution I found is mentioned above.