My system was installed about a year ago, but about 9 or 10 of the heads are starting to severely tilt in the hard, clay soil. As a result, I'm starting to develop brown spots from uneven coverage. Question: what is the best method for straightening the heads and keeping them straight? Is it just a matter of digging down to the pvc and placing rocks and soil around the stems, or is there some other recommendation? Thanks for your help.