dop2000 Changing my characters to a circle collision mask while keeping the crate's to collision polygon did solve the issue although the characters' movement got a little bouncy.
I still don't understand why one of the characters which has a square collision polygon glides through the crates - no bounce, no sudden stopping whatsoever. I have even made another character to have the same size, same animation, same collision polygon, same points and still it wouldn't work. It'll only work if I clone it.
Anyway this ordeal taught me a lot about the physics engine, thanks a lot!