All of a sudden our cold air returns don't seem to be working. The reason I know is the covers are clean as a whistle and I have an unusual amount of dust and debris on surfaces and hanging on walls and cabinets a la static cling. We live in the country on a dirt road and the amount of dust has always been bad, but I can't understand why the covers of all our returns are so clean and everything else is not. Didn't used to be that way. What could be causing this?