1) while in tavern, NOTE the lower RIGHT corner where it says MANA BANK. Press key it should in crease. ALL my characters this works when tested. If this is not working for you I have no idea what to tell you.
2) also while in the battle part of the game, you can likely enter the Defender's Forge where it displays Banked Mana and press the option and it rises there.
if this isn't working then i have no idea what to tell you. it works every time, with any character i have, as many times as i try it without fail. if you don't have ANY banked mana, then perhaps you need to have at least ONE or something.