Tank enemies use a special mechanic, called blocking.
While blocking, Tank enemies take no damage from attacks coming from the front.
You can still damage them by attacking from behind or hitting them while they are performing an attack.
After a Tank finishes its attack, it will resume blocking. However, attacking from behind leaves it vulnerable for a short time before it can raise its shield again.
While blocking, Tanks are immune to some crowd control effects, though they can still be Frozen or Stunned.
Landing enough Heavy Attacks will eventually shatter the Tank's shield, preventing it from blocking and creating an opportunity to deal significant damage.
To defeat Tank enemies efficiently, look for openings, attack from behind whenever possible, and use Heavy Attacks to break their defenses.