What mod are you using, or do you play stock SH3?
In GWX3 I have noticed also that surprising lack of aircraft attacks even in 1944, and yet in previous versions it worked very well. But when running a campaign from Penang, I get constantly harrased by aicraft when near the red sea, India, and any allied base, as it should be. Certainly very strange....
So far NYGM is the mod where I have gotten more consistent results (Meaning I'm ALWAYS harrased by aircraft LOL
) but probably that can be traced down to the fact that NYGM has "scripted" air attacks, not just random ones.