Some choice quotes:
QUOTE
The best description of this bug that I've heard is that new mana/5 enchants (shoulders bracers helm chest) tend to overwrite mana/5 that exists on your gear instead of stacking with it, or the highest is taken instead of both, they definitely don't all play well together. I've avoided all the mana/5 enchants for now, because they definitely aren't working correctly.
Blizzard is semi-aware of this, but even they aren't sure of the full extent of the problem. Just a heads up. This applies to more than just mana regen; supposedly all kinds of bonuses aren't stacking properly.
Other indications are that you ARE getting the bonuses, but your character sheet isn't processing them correctly and is lying to you. Mods that track your true stats, such as mana regen mods that gather data as you play, can tell you the truth.
-Bolty