I have found this job to be impossible. I have so many hard towels that I find myself having to buy new towels every other month.
Towels are expensive and even my most expensive towels, which you would imagine would stay soft for a long time, have gone hard.
I asked a few friends how they keep theirs soft and I intend to try some of these myself. 

“I use baking soda with vinegar to keep my towels soft. Pour these into the space where you would normally do your stasoft and washing powder. The quantity depends on the number of towels.”

“I stay away from stasoft and use vinegar or Woolite instead. The vinegar loosens the soapy residue and cleans your machine.

Others say cut back on the stasoft. It’s the residue that leaves your towels hard. 

Go easy on the tumble dryer.

An interesting one is to use a tennis ball in your washing machine, and another says to use warmer water. 

These tricks are definitely worth trying. Best of luck!