There are actually two separate issues that prevent George from using the hellfire. One is that after the first time, he no longer slaves the TADS to your helmet sight. He will do it the first time I ask, but after he successfully fires a hellfire, he won't slave again. That means you can't tell him to attack any other targets. I think I've found a workaround for this. If you short press W (up) he will say "roger ceasefire", and then he will slave again.
The second issue is that he will say "engaging" but will never fire. Like others have said, the constraints box does not appear. Again, this happens some time after the first few hellfires have been used. This problem seems to be unique to, or at least much worse, in multiplayer.
Here is a track from me playing on BuddySpike Blue Flag, Persian Gulf. The first flight, I'm attacking a FARP from a position in the mountains. George slaves the TADS at first, and shoots a hellfire. Then he refuses to slave again. He does say "slaving, deslaving", but the TADS doesn't move, or only moves a tiny bit and not to where I was looking. Then I tried the ceasefire workaround and he starts slaving again. Then I'm shot down by a jet.
The second flight in the track, I take off at Sharjah and fly over water to Abu Musa. George attacks a few targets with hellfires, then I start having the problem where he says "engaging" but doesn't. I'm in range and have a clear shot. Even if I didn't, it's a bug that he says "engaging" when he clearly is not engaging. I never got him to fire again, and I quit to desktop. This problem is extremely frustrating. The Apache is unusable in multiplayer.
https://drive.google.com/file/d/10cXVf8x2ur2H_Tyu0KSkQAD_oblV9L7a/view?usp=sharing