On the right side above the optical drive there are little plastic braces that latch the upper case to the main body. This is the reason why the upper case might stick here. If the above step is not working, also try to gently push the upper case sideways away from the optical drive after freeing all other sides. When reassembling make sure the braces are in their slots of the main body and not attached to the upper case. If you cannot fit the upper case evenly over the optical drive, that's probably why. In this case carefully remove them and slide them back into their slots before lowering the upper case.