Minders not spawning on vaults of umbrage?

CelticStones

Has this changed? A few guides and past experience say the minders are supposed to spawn at 75/50 and 25%. I've been stuck here for an hour, and the first 2 spawn on time, dome spawns, and then I carry on. I range the boss, get him to the next %age and he screams, no minders = no domes, = death. I've tried with and without ultis, slowly getting him to 50, nuking him to 50. What am I doing wrong?
Ok, It was the damn pet interrupting, :P ignore.