I have Egyptian cotton towels (Christy's) and they're still lovely and fluffy after many many washes. I just use a standard laundry liquid and tumble them on a cool (not hot) setting.

I wonder if you've got a build-up of detergent in your towels that your washer is not rinsing out?

Check first that you're not overloading your washing machine - if there isn't enough room for the water to circulate properly the rinsing won't be so efficient. (For a towel wash you need to leave plenty of 'headroom' in the drum.)

Then try putting the towels (clean) in a wash cycle without any detergent, so that they get a really good rinse.