It may be the shape of your carboy neck. I've had the exact same problem with 2 of my 12 or so carboys. They are the same no brand 6 gallon size. None of the other carboys do this so I know it's the carboy and not the bung. The sizing is also important. Some glass carboys take a 6.5 stopper and some take a 7. I think the 1 gallon jugs take a 6.5 and the 4L Carlo Rossi jugs take a 6.0. Unlike some here, I like the silicone stoppers more than the gum rubber type, and the silicone hoods are the best.
Anyway, I got tired of the lack of standardization and have basically switched to Stainless kegs in 5 gallon and 15.5 gallon size. And with those, I use exclusively the silicone waterless airlocks.
The only problem I would have with stainless kegs is the same I have with my barrels, you can't see where to stop the racking cane to avoid the sediment. How do you deal with this?