Not always is AP the answer. When ever I up armor, I generally take a mix of ammo, depends on what I'm going after.
Jeep, M3's, M16's, M8's, LVT's, & Osti......respond better with HE. I've found that even the heavier tanks can be effected by hitting them in the tracks with HE, but generally HE won't kill them
Heavy armor there are weak spots in each of the armored vehicles. The trouble is, these spots are small, and take much practice to easily acquire, much less hit the "Sweet Spot".
Rounds bouncing off: Keep in mind, AH isn't some simple arcade game. HT & staff devoted much into realism. If you have a round bounce, then look at the big picture and learn: how far was your target, did you have to elevate in order to range your target. If so then your round came in at a steep angle, which means more times than any your round will glance off the side of the armor.
The best shot is when your can deliver it to your target, angled at 90 degrees to the target surface. The more your off the 90 degree mark, the more you'll experience rounds bouncing off. Easier said than done lol, time and practise.
Scca if you'd like more info on those "Sweet Spots", PM LTARkils and he'll hook ya up