You've got basically a couple seconds at most to click on the little tactics icon that pops up (on pc) in the bottom left of the screen, just above the quick slot bar. If you don't click in time, since you are generally casting after zoning, zone back out then in again, then try again.
Which pet summon mod did you use? I have it available on DAMutator as one of the options, but to be honest it isn't one I chose to use. I just have little need of it. If you tried something else, then I've used the mutator with no hitches so far (I've had it well over a month), though as I said I didn't choose the pet summon retaining thing.