Two things I can think of trying.
1. Get the o-ring juicy and gently seat the glass down on to it. Sometimes a juicy o-ring is needed to help set the proper pressure for the glass. Also having it juicy might let it slip back into the groove as you sit the glass in.
2. You might have gotten the o-ring stretched, I don't know for sure you did or didn't. Not blaming here because I've stretched enough of them at times as well, and not even realized it until, ... So, you might need to use any of the spare o-rings the kit/atty came with.
Hope I've helped.