It’s called major atrophy. HCG will help it. 2000-2500ius should do the trick. Mine get so small one is gone. Not a good feeling. What are you running?
My left one has done that my whole life , retracts during sex then drops back down after I relax. When I had my vasectomy the Dr said that the vas deferens on that testicle was a bit shorter and asked it it ever retract inside me. I explain it's always done that and he said it was not an issue at all and is not that I common.