If you have The Immortal Sun in play, you can’t trigger cards with the Awaken ability. It’ll fill up with extra mana but never provide the bonus when cast. It’s probably checking for the total mana on the card and not factoring in the cost reduction.
Awaken effects may be broken in general.
It’s being discussed in this thread: Awaken effects broken [Investigating] - #3 by relax - MtGPQ Bugs & Technical Issues - 505 Go! Official Forums