What are the requirements for blocking an enemy attack?
Solution 1:
Parrying on lower levels works only when you are facing the enemy.
The Level 1 of the Parrying ability from the Training tree unlocks the ability to parry blows from all directions, not just the one you are facing.
Regardless of this, timing and sufficient Vigor points are prerequisites for parrying.
Solution 2:
You must have enough energy (vigor) to block an attack. Vigor is used for both, casting signs and blocking/parrying attacks.
I use hit&run tactics mostly, blocking seems to be useful only in 1vs1 battles.