I experienced the same symptom before. A quick and easy workaround: quit the qt program and restart it again. Then it says there is a X hours of out sync, and tries to sync again. I guess it is a problem bug due to small amount of nodes.
previously I had the same problem. after a while the qt wallet stopped syncing unless a restart is performed. I remember 0.8.5 litecoin wallet has a bug which stops online activities, and I am not sure if pts wallet has the same bug (but the problem looks exactly the same).
but for the bug that I report in this post, the problem is different: even I restart, the wallet cannot sync and it shows no node connected. Recently the problem is solved. It seems that this time the problem is not on client side, but on node side.