Incident? Hmm...
Cass could manipulate water just fine. That didn't seem to be a problem. Maybe all of this second guessing and low self esteem was from that incident she had years ago? It was very possible. Perhaps something happened while trying to manipulate fire... probably something really bad. Maybe she felt like something similar would happen again.. and even if she felt like she was letting it go, her mind might be stopping her subconsciously because of that event. Either way, Ryan had to make sure he wasn't too hasty. Cass had to learn to trust herself with her powers again, and if he pried a bit too fast, that could possibly never happen.
Ryan let a gentle breeze flow through the room, and the water slowly started to dry up. "Cass... You mind talking about what happened years ago? If you don't want to, I'm fine with that too. Only tell me if you feel like you want to."