3rd and 6

I have noticed in our last few games(along with many of you im sure) that we struggle to convert our 3rd and mid yardage situations(such as 3rd &6,7,8etc). Often times we complete passes that are 4 or 5 yards leaving us short. Why are the recievers/tight ends routes not all ran to the sticks. If we would run our routes to the sticks then we would be able to convert many more of our 3rd down opportunites. I dont understand it....