Good control valves are fairly expensive, around $300 (Klack and Fleck
I think are good ones) so people who want to make the most profit buy
cheap ones that last maybe a couple of years and mostly need to be
replaced instead of repaired.

Simple troubleshooting list.

Is there salt in the brine tank where water can reach it?

Is water going into the brine tank?

Is water going out of the brine tank?

Is regeneration happening? (timer working) Water going down the drain
pipe.

If there is a manual bypass, is it set correctly?