A lot of times, if the lock mechanism is working (and if there is a difference between how much you can move the glass when it is locked or not), the top channel on the glass has come loose and is jamming things up. When you look at the glass, you will see the lift channel at the bottom, and then side channels, and a top channel. These basically clip over the edge of the glass. Have a really close look at them all, especially at the top.