After time, with many washes and use, towels start to build up detergent, fabric softener residue and because of this won’t allow your towel to absorb as much water, and start to smell.
Here are some steps to follow:
Wash your towels on normal cycle with only a cup of vinegar,
And then run again this time with a half-cup of baking soda.
That will strip your towels from any residue and mildew smell and will actually leave them feeling fluffy and smelling fresh again.
Do not add laundry detergent to either wash. Just once with vinegar and once with baking soda.