It sounds like you're not training hard enough or are not doing the correct training for your goal.

Are you specifically training for size gain? Are you doing the big three (bench press, squat, deadlift)? Are you training your muscles to complete exhaustion each set - which means that your last rep is the one you cannot physically complete? Are you increasing the weights if you're hitting the target reps on the first set? Are you taking rest days and getting enough sleep?

Honestly you should probably talk to a trainer at your gym, they'll be able to sort you out. I can also highly recommend ATHLEAN-X on youtube (https://www.youtube.com/user/JDCav24), he really knows what he's talking about.