Some guys are simply secondary. Happens sometimes near age 20. So not always age related. Sometimes a HPTA restart works when not an old guy. There is a thread for that. https://forums.t-nation.com/t/hpta-restart-for-trt-w-application-to-gear-pct/211723
If not caused by prolactin or high E2, there does not seem to be much that can be done. Sometimes a MRI can show damage to the pituitary from a blow to the head or a adinoma. If an adinoma makes prolactin, that can be fixed with Dostinex/cabergoline 0.5mg/week. Otherwise no probable action to recover LH/FSH.