That can't be answered completely with the info at hand, but I can give you the idea of the background.
What airplanes do you mean specifically? Neither Ki-27 or P-38 can divebomb at all.
Of course only those classified as divebomber in their classfile can divebomb at all. But the bombing behavior pattern the AI choses, is based on complex conditions.
The GAttack waypoint altitude is one of those. That waypoint must be above a certain level (2000 or 2500 meter iirc). If it is below that, the AI will chose a low level fighterbomber style attack (not levelbomb, for dievbombers)
Big levelbomber have the same principle. if the GAttack is too low, you can watch B-17s precision bomb in a shallow dive and not level bomb.
Now you have to understand how DGEN works. It is a randomisier that picks from certain database files and mission templates (all in the DGEN folder). If these define such waypoints at low altitude only for the given campaign -> No divebombing.
IIRC the plane type can be assigned roles and when the plane you are watching is not assigned a divebomber role in these databases for the given campaign, they will most likely never get added in that role with any of the specifics for the mission parameters addressed by DGEN.exe.
So, a solution would only be someone to dig into these files and analyse and modify. That may or may not produce any desirable or undesirable effects.
It is not a bug, it is not a BAT specific issue, it is something that's around as long as IL-2 exists. DGEN basically is acting as a game within a game
PS: Most (if not all by today's standard) of the DGEN campaigns and all those databeses for them are the work of modders in one way or anyother. So they reflect the knowledge and abilities of those modders at the time of their creation.
(just in case you think this all should be perfect

)