Device-to-server sync issues with Android device

I seem to be having the same problem described earlier in this thread. Updates in Zimbra Calendar get pushed to Android phone with no problem but it does not seem to work from Android phone to Zimbra Calendar. I have tried manually synching from the phone and it does not work either. I'm not sure what to try next. Any suggestions?

ZeXtras version 1.8.12

Thanks,
Kevin