High-Low Blocking

I'm finding sometimes when I block standing for a standing attack I still get hit. And other situations where I would expect an attack to black but it gets broken through. Is this normal or is anyone else finding blocking between crouching/standing attack/block system a little confusing. For example if I crouch block a jump punch towards me breaks the block and opens for say an uppercut right after. I'm use to it i'v found ways around it. But other situations i'v been in it doesn't happen.

you have to learn what hits high, and what hits low, if your using the right block it should never get through.
some people have combo's that begin or end with low hits that'll get through.

if you wanna block someone jump punching towards you, stand and block.