These can be seriously tricky to work out, but since you only removed the top case, that should narrow down your investigation. Start by checking the lines of the machine. Make sure everything is straight. (You'd be amazed exactly how many different ways these machines can get bent).Check the gap all the way around between top and bottom case. Especially along the back edge, the corners by the hinges are susceptable to catching and bending upward. Examine the latch mechanism. Make sure the screen is centred over the bottom half and not overhanging on one side.
Check the hook is coming out at the screen end using a magnet. Catch it and check it can move as far as it needs, check it springs back correctly. Check the latch on the top case (actually its part of the bottom case), pressing the button as you do. You should see the part on the right hand side which the hook catches on. I have seen these break. It should be squared, and should obscure one end of the slot entirely, about 3mm or so.
If nothing seems out of order, its worth removing the top case again. Check its nice and flat when removed, then go around the underside at the edges and straighten all the clips. Check in the bottom case for cables which might be getting trapped, again near the hinges is a spot where this can happen without being obvious. Also the left hand side tends to be prone to cable pinching, since the bluetooth board is at the front there.
Hope that helps.
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