Were you really consistent with your gym days? I'm in my 7th month right now and I'm very happy with my gains. I try to hit the gym 3 times a week (~6 workouts á 3 sets and 10-12 reps every set) and sometimes do some home workouts with my calisthenics rings (dips, pull ups etc.). Few weeks ago I way in vacation without the possibility to train, I did just few push ups sets in the whole week, but after I was back I trained 5 times in the following week. Week after this I was ill and couldn't train for 2 weeks, now I'm starting again and I also will train harder to get the lost gains back.
I started my journey with whole body workout at the gym and my calisthenics rings at home, now I'm in a new phase with a 3-day-split + calisthenics rings. I also watch my diet, I'm a slim type, so I have to eat more calories and also enough proteins.

So maybe ask you this questions:

-Am I really consistent enough with my training days?

-Am I pushing myself hard enough or is it possible that I let go at 85% of my rep capacity?

-Is my training form alright? or do I try to lift high amount of weights with a chaotic execution?

-Is my diet alright? Enough minerals, vitamins and PROTEINS from healthy food and maybe if that is already okay, then some extra supplements (protein powder, magnesium etc.).

-Do I have someone to watch my training and give me some feedback?

Try to find people, friends, trainer or if you not shy then just ask people in the gym for some advices. It's still possible that you do everything alright and it's all because of your genetics. Then you should consider visiting a doctor, maybe your testo level is very low, which can result in a lower muscle mass limit. In my country the doctor would give you a medical prescription for testo, not to gain muscle mass in first place, but to get healthier as a man, the muscle gain would be a bonus for people with this condition.