That's good that you're seeing gains, but it might be that you're not getting enough protein. As a rule of thumb, you want to consume .7-1 grams of protein per lb. of bodyweight, and the leaner you are, you should have closer to 1 gram of protein per lb of mass. For example, I weigh 250 lbs, but I am not quite as lean as I should be, so I eat about 175 grams of protein a day. Have you tried changing things up, like trying new exercises? This 'shocks' the muscles, as they are not used to doing the new movements, or you can up the reps, and lower the weight. Or, maybe you can do partial reps, where your arms only go up until your upper arms are inline with the bench you're on? This places most of the stress on the chest. Regardless, good luck, and eat big to get big!!
-John

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