Same as https://forums.bitfire.at/topic/2136/lists-syncing-contents-not? Can you provide logs? Without verbose information, we’re not able to say “why”.
I have been using DAVdroid for a long time (thanks for developing this very useful tool!), but I recently noticed that synchronization with my server had stopped working for over 2 months without any warning. The underlying issue was entirely my fault (the caldav server software was no longer up), but I was surprised that I didn’t have any warning from DAVdroid about the fact that sync had been broken for so long. You can notice this in the setting (which gives the date of the last successful sync), but you have to look for it to make sure.
I think it would be nice if DAVdroid notified the user if sync has been unable to complete since a long time (say, 1 week, but ideally a configurable delay).
What do you think?