I just had this same issue just the other night on my first attempt to brew anything (Eds Apfelwein)!
After everything was all set, I put the stopper/airlock in and it would just slide on up.
Did this a couple times and figures it had to be some of the Star San sitting on the plug and bung.
So, I placed plug on a paper towel for a few minutes and put it back with no sliding issue.
Originally Posted by AFAJ Brew Guy
Just be glad you didn't push the stopper all the way through like I did!
When I bought the carboy, the LHBS peeps warned me about that lol!
What would one do if it did happen I've been wondering!?