It depends upon the level of enemy damaging you and how much they attempt to damage you that will give you a percentage back. If you just have dregs/psions shooting you, you'll get next to nothing. A boss, a good bit. This is pretty much my main class I run and they do work just fine and as intended. Also note, if you are in Crucible and someone hits you with a Disruptor void perk, after a few shots it will knock you out of your super with nothing regained.

Its not bugged, the ability gives you a certain amount for how much damage you block, it's not always going to give you half energy because you blocked a few shots from a psion, this is intended to have the same effect as the Shards of Galanor nerf