-- You may have reached a "plateau?" I don't know what a plateau is! According to Webster's, a plateau can be several things. We'll go with definition #2 which seems to fit here: A period when something does not increase or advance any further; to stop growing or increasing. Definitely looks like definition #2 fits the bill!
The easiest way to get off that "plateau" is simply to change your workout routine. As Arnold would put it, you need to "shock" your muscles in order for them to continue to grow. Do you go to the gym and do the same workout in the same order every time you go? If so, try changing your routine on a frequent basis. Most experts recommend a change at least every couple of weeks. One time, you may do hammer curls when you first enter the gym and then you go onto the bench press and then onto dips for the triceps.
Your body will get used to this routine and try to figure out how to compensate or accommodate what you are doing to protect itself since you are tearing it up. Change it up! Next time try doing dips first. Then possibly continue with your delts as your second exercise for example. Continuously changing up your routine and keeping those muscles confused or "shocked" will help them to grow.
I, personally, like to do a full body workout (as taught by Joe Weider), three times a week but I never do the same workout two times in a row. One day, I may do legs first, then the next workout, traps first and then again on the next workout, lats first. Change the order, change the number of sets, change the number of reps and/or change the weight. All of this will confuse or "shock" your muscles and you should start to notice a gain once again.
-- To increase muscle mass, you will also need to increase your weight? Not your body weight, but the weight you are lifting. This is the unwritten first priority of all bodybuilders and can be one of the reasons you are not gaining the muscle mass you are looking for. It's a proven fact and the main building block in the sport of bodybuilding.
The "rule of thumb" used by most bodybuilders is to increase your weight on any particular exercise when you are maxing reps in each set and not feeling complete exhaustion of the targeted muscle. You should be lifting heavy enough where you can't possibly finish the last rep or even two. This should give you a gauge where you should be. Once you can lift a particular weight and complete all reps "without a sweat," it's time to increase. Bump it up! This will most likely happen approximately every thirty days but then that will vary depending on your workout routine, frequency, how long you've been lifting, etc.
So there you have it! A couple possible causes of not being able to gain muscle mass. There are more reasons so if you need additional help in this area, a little research will give you a few more options, but these seem to be the main causes.
One word of caution, if you are working out (lifting) alone, you should not attempt to "max out" in any exercise that may cause bodily harm if it cannot be executed and completed in a safe manner. Enjoy your workout but please be safe in order to lift another day!
See YOU in the gym! TEAM JAQT And don't forget to order your supplements today!