Help with the underhook?

I’m a left leg lead and like to undertook with my left arm. I used to be able to whoop guys with it but I got injured for a while and somehow can’t get it anymore. Or/a mix of I’m at that point where you start realizing your trash and actual technique starts developing instead of button smashing spazziness. I can’t wrestle this year because of some family stuff, but I’m able to do no gi bjj once or twice a week. I’m the only minor in the gym. Everyone is stronger, heavier, and taller. I can hardly hit takedowns on them. I really like the underhook but it doesn’t work that well.

I can’t seem to move them to set up knee picks or snap downs. I can’t get them to step, tips??? Would an underhook just not be the best option against these stronger taller grown ass guys?

I’m attempting to get good at the body lock too from my underhook. It’s really scary to attempt the body lock on these guys because many of them know judo throws or are former wrestlers. My coach says i need to crunch to get them off balanced for throws and trips. It’s scary and hard. I asked him if a body lock would even be good for bigger guys, he said it still would work. Tips?

All the videos I watch don’t really help. I can’t move anyone that much with it. Anyone have any videos that go into deep detail on the underhook and body lock. Any tips?