Why does my grass look thin?

When we first moved in our house, there was alot of weeds. I sprayed some weed killer that attaches to the water hose twice. This year I sprayed a whole bottle on the front yard and put weed n feed on it. I usually just put a half bottle on the front with nothing else. The grass looks good now but its not as thick as the grass in the back yard. Do I just need to put grass seed on it? It is bermuda grass. BTW this is my parent's house. I am about to graduate. I just do all the maintenance and repairs around here.