And you actively have to use the block action (right click per default setting) when your opponent is attacking you.

Just equipping the shield is not enough.

I can block attacks from both kinds of troll and even abominations with a lvl 2 bone shield and lvl 3 troll armor. Some damage is coming through but your selfheal should be able to mitigate this.

Of course, as @blprice61 already wrote, good food is of utmost importance.