I've had issues on WP 8.1 and 8.1 DP where the lock screen will gitter when attempting to unlock phone by sliding up. It doesn't always happen but enough where its worthy of complaint. Anyone else?
download lock screen BETA and give it a try. if problem still persist what about a hard reset... if still not solved. its best to handle it to service centre
I'd recommend NOT trying the Lockscreen Beta yet- look at the forum thread on the app bugs and you'll see that even after uninstalling it there are glitches.