I have the same problem and have tried to get an answer to this but I don't think there is one. Some it can help if you log out of your account and log back in. Switching between logging onto Facebook and king can work for a while but the problem always returns.
Another change for the worse, harder tasks, rubbish rewards. Now I've found you can no longer go back to an earlier to complete tasks as you have to complete the level to keep whatever it is you're collecting. The chocolate box is not worth bothering with any more.