Well, go to the place where you last got her. Like in the Jail. I always find her in the jail ^.^ and also, You need to buy Flowers from Yuna in Welling before any battles with her or she'll run away again XD
I've lost Roca for over 3 times so I've wasted 3 stars D:
User : Sonata
Server : Scorpio
Lvl : 5x