Not being able to activate a muscle you know is there? That's how I landed the rollover drills- took forever to relearn how to activate my obliques as part of that motion. Good luck finding your glutes!
My deltoid, though in this case there's a dead nerve to blame. Anders A is an expert at activating muscles. I'd fly him in.
Tiger Woods had that problem, too.
Several people has told me he is pretty good in it he does. I don't have personal experience though (other than noticing couple of times my muscles would get pretty seriosly activated if try keeping up with him in forest).