Every one in a while when I wake up my phone and enter my (correct) pin, it will sit on the pin screen until my phone falls asleep. Waking it up, I'm back on the lock screen and have to enter my pin all over again. I can keep pressing the number pad to keep it awake, but it still sits on the unlock screen indefinitely until it falls asleep again or I press "back." If I enter a wrong pin that's four numbers or more it does the same thing, but when I enter in three or less it says "wrong pin." The only solution that works is turning off the phone and turning it back on again, and I'd rather not do that.