Between myself and the neighbor, we have at least 6 No-Spill cans of various sizes. The only leaks have come from cracks in the black plastic base that attaches the cap to the can. It can definitely crack from being tightened too strenuously and perhaps from excessive pressure built up if sealed can is left in a hot location. Every one of them start and stop pouring just as advertised.
Regarding the OP's problem, I'd use a flat piece of material to check that the top of the can opening is perfectly flat so it can make a tight seal with the o-ring. If not that, elves???
Regarding the OP's problem, I'd use a flat piece of material to check that the top of the can opening is perfectly flat so it can make a tight seal with the o-ring. If not that, elves???