Can't sync since Connector 5.0.8 installed in June
Why? It worked well on Firefox before I had to install the new connector. When I press the sync icon, it rotates and a blue square icon comes on to the left for a few seconds. Cursor on the rotating green icon says...not yet synched...
http://zotero.org/download
Make sure you're running Zotero 5.0. It sounds like you're still on Zotero 4.0.