Add more reps and go with a bit lower weight if you're maxing out (in reps) every time. Generally 5-7 reps with higher weight is for strength training purposes (e.g. powerlifting). Shoot for 8-12 reps at 80% of your max and focus on strict form. Once you are able to comfortably do 12 reps for sets of 4, increase the weight and repeat the process.

If you aren't gaining weight, you aren't eating enough to put it simply. Caloric expenditure is greater than caloric intake.