So, I got you right that some AI combatants knocked you down with their shields, right? Therefore I guess you mean the shield bash. Here is how it works: You first have to click right mouse button and hold it down, than click the left mouse button while still holding the right button. Hold both of the buttons down for an instant then release the right button. You may have to practice it a few times until it gets fluent and not confusing anymore.
Maces as well as two-handed axes and/or long-axes, hafted blades, the baltic big badass clubs or a glaive are all very good for cracking thick armours. One-handed axes are also not bad albeit not as good as maces. I would prefer an unmodified standard military sickle over any unmodified standard 1h-axe - the moorish axe might be an exception to this. However, my favourite against well-armoured opponents is the studded club (2-h weapon, 38b damage, 92 speed, 92 reach), it's almost always a 1-hitter against everybody (knights sometimes may need 2 hits though). It can crush through blocks with an overhead strike and it's great for taking prisoners. Also, maces and other weapons that do blunt damage (hammers, clubs, maces, etc.) have the capability to knock people down.
As far as fighting on foot is concerned - if my enemy is consta-blocking with his shield or his weapons he gets clubbed once on his bulb with an overhead strike which normally crushes through and immediately afterwards I try to hit his legs with a second strike. Since the armour values of helmets are sharply increased in this mod it's almost always better to go for the legs as they are normally weaker armoured than the rest of the body. You have to consider your proficiency and character level, though. My current char has a prof. of 462 in 2-h weapons with a power strike of 10 so he has no problems to strike through blocks or generally hitting hard and fast with almost every weapon. That means, when my char was younger and had some 200 prof. and a power strike of 6/7 he relied a little more on quickness and/or reach of weapons and was equipped with a heavy kite shield, a military sickle, a glaive and some balanced 1-h sword. The glaive is also a fantastic weapon for fighting horsemen or fighting in a siege defense.