ramsesisthebest

This might be a really stupid question, but I am having an extremely difficult time placing plates on top of each other. Whenever I go to place one on top of another, it simply goes underneath it instead. I'm new to LDD, so chances are there's a really simple explanation to this and I just don't know it, but I haven't been able to find anyone who has the same problem. The project I'm working on wouldn't take all that long if it weren't for the fact that it's taking so long just to get each piece placed correctly. I'm really sick of being unable to get any work done on it because of this stupid problem, does anyone know what I'm doing wrong?