Putting fantasy geek hat on; I can only assume said dragon is "very ancient." We're talking beyond ancient status here, because even ancient dragons are prone to slices from human (dwarven, elven, orcish, what not) bladed weaponry (game system depending).
Or, said pilots just happen to be untrained, or ignorant enough, or unlucky enough to keep hitting the dragon's topside reinforced armor.
I wonder how a grav tank or grav attack "gunship" might fare. G-carrier with an X, Y, or Z gun on the pintel mount? Empress APC?
Switch it up a little, and maybe the dragon has some kind of super-reflective armor. Suddenly those heavy high energy weapons are splaying off a mirrored dragon with god knows what as a breath weapon.