Would suggest you to launch Lightroom CC on your machine and click on the drop down menu on top left just below your name and make sure syncing is turned on.
If its turned on, check on Lightroom Web if you have any collections over there.
Lightroom Web link : https://lightroom.adobe.com/
Also, if you can share the workflow of syncing that whether you are trying to sync images from Lightroom Mobile to Lightroom CC or vice-versa?
This corrected my sync issues however I must admit Adobe did a great job hiding the sync images switch. Why would they place this under my name without any instructions as to where it is located. I can't tell you how long it took me to discover this and calling adobe for support doesn't exist much today. Great product terrible support.....