I agreed with newsue - sounds like soap scum build up. Use her hot water and vinegar treatment though it may take a couple of passes to get all the soap build up out of the fabric.
It could be several things - first, check your bleach dispenser to make sure it's not blocked. If that's not it, then I imagine the problem is that your towels have accumulated a soap film and need to be "stripped".
Wash the towels in HOT water with 2 cups of white vinegar and run through two rinse cycles. If that doesn't help, repeat the process. If they are still dingy, and the bleach dispenser is clear, you might have a problem with your water. Good luck!
Add your voice to the conversation. Click here to answer this question.