Ashe double seraphs doesn't funcion

I put a Seraph's Embrace on Ashe, and she got 20 mana each time a spell is cast. I put a second Embrace on her, and suddenly she stopped getting any mana at all except from her autos. Not only that, but occasionally she would LOSE about 20 mana when a spell was cast.... (no, there were no Kassadins around, so it wasn't being stolen).