Hi, I've couple corny. One has an opening slightly larger than the other, about 4-5mm. Both are the same brand etc, Always finding it troublesome to seal etc. Anyone have similar problem. I've had to tighten the close by propping coins under the lid closure. Trying to search for larger diameter o-rings but hard to get here.
There can be some variance in lid and ope sizes and some effort required to close.
There are a number of manufacturers, some are better than others.
If you find that a seal is not quite sealing, put it in place as best you can, invert your keg and bounce the keg handles off the ground.
The weight of the beer should seal the lid.