It’s difficult to tell, as this varies from person to person. It could be that your training program is not difficult enough for you, you aren’t eating enough to fuel growth, you aren’t letting your body recover between sessions, or you are doing too many other activities outside of weight training. It could be any one of these factors, or it could be a combination — just take a look at your nutrition and fitness regimens, and see if any of these factors stand out to you as an area for growth.