I am pretty new to kegging myself, I actually just posted a carbonation question. But I have watched a lot of videos, and read a lot of info about the corny kegs. Sometimes it can be difficult to get a good seal when putting the lid back in, and sometimes it may even need a little bit of pressure to help push the lid completely tight. I would just try re-seating the lid a couple of times.
If that does not work, check the new seal for any cracks or breaks, and then try putting it under a little bit of pressure. just enough to help push the lid up and get a good seal. That is what I would do...however, as i mentioned I am rather new to kegging as well. But I do know, my lid was stubborn as well, it took me a couple of times of positioning and clamping it down before I got a good seal.
Hope that helps,
EDIT: Looks like Mr. Google beat all of us to it!!