I've never had a problem with the lid itself, but I always have problems trying to get the neck off the main body (for cleaning).
I always need to pull the skimmer out of the sump and grab it with a towel to get some extra grip...take a look and make sure your turning the right way too. The lid should be counter clockwise (the same way the top portion turns off the neck).
If you do get it off, you might want to use a Dremel to make it a little easier next time around. The lid itself does not need to create a seal...