When monsters undergo Crystarium Expansion (ie.e. when they hit a certain level), they start using different materials to level up. Your Cait Sith probably hit that point and now needs a new kind of material, so you can't use your old stuff anymore. If memory serves, you should need to start using Sliver materials now (As opposed to the Droplets you were using before).
I'd keep Cait Sith at 15 (don't other leveling it more), it sucks for mid - end game. Just use a SEN like Pulsework Knight to take hits for you. Noel and Serah get the Medic role early too, so there's that.
Deal with it- http://tinyurl.com/amdcu8s FF XIII/Skyrim = 100% -- GT: Gears of Wario Now Playing: Borderlands2/FFXIII-2/PMD Blue Rescue Team