Lack of consistency
Just turning up to the gym here and there isn’t going to cut it. You need to structure and plan your training days (and times) and stick to them like glue.
Regardless of what your training program setup is, one thing is for certain that with the inclusion of adequate rest in between training sessions, the more you can train a muscle the faster it will grow.
Some of us may be able to train 5 days a week while some of us might only be able to train 2 to 3 days a week, everybody’s situation and schedule is different. Whatever your availability, you need to be smart about how you program your training.
For example if you only had 2 days to train in the gym, choosing 2 full body sessions or a upper and a lower split session would be advisable over a single chest session and a biceps and triceps session. Both sessions should consist of big movements like the bench press and the squat, depending on training experience and certain mechanical factors. Be sure to include some unilateral movements(single leg or single arm).
Pencil your training into your dairy every week and never miss them for any reason. Consistency produces results.