Lest you're point-blank and completely flat onto the target surface, the period HEAT ammo wouldn't detonate due to fuse design. The game models that by not normalising the stuff at all.
Also, by that tier, a whole lot of the vehicles are pretty much designed to be ****ing impossible to trigger a heat charge on their surfaces (see above,), have an extra tank's worth of spaced armour all around their hull (IS-3 to 7, though they also do the previous thing), or have armour so thin (because it was pointless) you'll be itching for an HE shell anyhow (Leo, AMX-30, Jappy-Jap-Japs (In particular, Type 61, I think.))