Syncing apparently stopped working

PhDinHistory
I am using Zotero 2.0 on a Mac, which I just started this week. Previously I was on a PC. Not sure if that makes a difference. I read that syncing is supposed to happen automatically. This process seems to have stopped for my library. I added a couple dozen journal articles today. I expected at the end I could just click on the green circular syncing arrow. However nothing is happening when I try to click on it. When I hover the pointer over it, I am told that the last sync happened 16 hours ago. I checked my library at zotero.org and confirmed that the two dozen articles had not been synced to the zotero servers. Do you have any suggestions? Am I doing anything wrong?

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