You telling us you do bench presses with only 15kg on each end of the bar???? If you are using an 7 ft. Olympic bar that takes the plates with 2 inch holes in the centers and only adding 15kg. per side, you are only benching about 115 lbs. and if you are using the smaller bar, you are only closer to 100 lbs. If so, there's the problem. More weight, fewer reps help develop bigger muscles. Lower weight, more reps keeps things toned.
You want to be doing about 5 sets and 3-5 reps on your bench. Train for power and keep increasing your bench by 2.5kg every time you go into the gym and do the exercise. You also won't grow if you don't eat. It's not really complicated to build a chest, you just need to put in the hard work and again eat eat eat.