FWIW, in my time on this board I've seen this issue pop up a lot, and I've had it myself. While the causes vary, they're usually not too costly to fix. The challenge is in finding the problem.
Sorry I can't be more helpful, but you'll get some great expert advice here. But if it makes you feel better, it's a common problem with a usually simple solution -- although it will take time and effort to identify the source of the problem.