To add:

I've cleaned and lightly sanded the mating metal surfaces just a bit (just enought to expose a shiny metal surface- very light duty paper), wiped down again, and also ensured visually that both surfaces lay as flat as possible to each other prior to putting the gasket sealer/ gasket

Upon install, there may be a little play to the right/left/north/south when mating the trans to the pan, again I ensure as close a match to the lips/mating trans as possible

fyi- the gasket sealer I observe merely holds the gasket from moving around too much as the gasket is usally plenty enough to seal

I've seen trans pans that were inadvertantly kicked or stepped upon/ lip bent when off the car, and it affected the proper trueness/ straightness of the pan's leading lip.

Upon thinking, perhaps the trans filter inside is preventing the pan from sealing up close enough to the trans...bad trans filter or improper for application?


...just a few thoughts, hope this helps
-fad