Could be that the angles are off slightly due to a set-up error when manufactured, or the end rails on your bathtub are holding the intake up off the heads a slight amount.

I've used .015 paper gaskets glued to both sides of the tub for decades, because I don't want to chance an air/oil leak and have to do the job twice.
Sure, a lot of guys get away with even re-using an old bathtub, but my luck's never been that casual