sometimes those gaskets don't seat real well. Oddly enough, I also had one that swelled or something... OEM gasket. It developed a 'burp' in the o-ring and wouldn't settle into the groove at all, being probably 1cm too long (!) so I replaced it with one of my keyster gaskets and no problems.
In a real pinch, you could use a TINY thin layer of a GAS PROOF sealant/gasket goo to seal it up. I really stress those three words because any extra might get sucked into your jets (because carbs hate people and will do that crap on purpose) and 'fuel resistant' only means like "until you get the carbs on". I had 2 extra rebuilds for those reasons.