maybe I know what the prob is, I have that with my modded 2.2 tablet here. When it's locked, briefly the lockscreen appears, I have to push on the lock and slide it down, sometimes it just doesn't work that fine. You however can alter the way the locking behaves, check under Settings! Good luck.
I had the same problem. It drove me kracy!! Very simple fix:all you gotta do is connect a mouse via usb thru ur u disk . Once ur connected u will see the mouse on ur screen simply slide lock. U will need to calibrate ur screen and problem solved!!!!