Just wondering, are you using a Drying Aid product of some kind during the drying process?
Did you recently make a change in products?
A Drying Aid or Car Shampoo (that leaves a wax or protection of some kind behind?)
I do use a Drying Aid and for that reason I wash my MF after every use, even if the towel appears clean, to avoid build up of product.
Also keep your Drying Towels damp after use until you wash them, don't let them dry with product still in them.
I also don't use a Spray Wax during my drying process with my good drying towels. Use a dedicated towel for that.