I agree with Rugby. There is not really enough information to give a certain diagnoses, but 20 minutes a day is not much for watering, and a weakened lawn will draw in a lot of problems.
Most chemicals that go after chinch bugs will not have much of an effect on grubs at least in the same application. Grubs are feeding down by the roots, which chinch live on top of the thatch layer. So one you need to leave the insecticide along after applying the other needs to be watered in to get down to the grubs.
A preventative application of Merit in the spring
will cover both problems though.
Right now is not the right time of the year for a de thatching,(spring
is best) but you can do a core aeration in the fall to help your thatch problem.
Actually thatch is a good thing for a lawn to have. It is only when it gets to be too thick that it becomes a problem. Up to a half an inch of thatch will help control weeds
, and act as a blanket protecting the roots from extremes in temps, while allowing oxygen and water to pass through. More then a half an inch will start causing you problems as it will act as barrier and hold the moisture, this will in turn act as a breeding ground for diseases and insects.
I hope this helps