take the frag plug off and super-glue the monti to the rock. If that won't work, take some reef safe eco-putty and put that down with some superglue on the putty and that should hold it in place.
For me, sometimes it's just a feel thing. I slap some glue on there and have an idea where it's going to go, then I just move the lug around until it seats in a nice little crevice. Hold it there with some twisting to get the superglue to get into the rock and it should stay.