Leaking at transmission filler neck... lil help?

I have a small but consistent leak where the filler neck attaches to the automatic transmission. The o-ring on there was pretty worn so I replaced it with a new one of the same size but it's still leaking about the same. Question is, is there a specific part that I should be using? Or is there a specific gauge or size of o ring that needs to go on? Does it just need to be seated somehow that I'm not doing? It goes in mighty tight but still as a little flex to it which seems like to how it should be but it continues to leak slightly. Thanks in advance as always!