Without knowing the specifics of your water conditions I can only offer possibilities...

Iron in the water can foul the resin and will result in a pressure loss.

Chlorine in water can turn the resin to mush over time and that will cause a pressure loss.

What are the details of your water conditions?
How long has the softener been in service?
Has any routine maintenance been done on the softener?

Keep the softener bypassed until this problem is resolved.

This softener has been in service since 2003 and the city water hardness is 20 and the hardness is set at 25. I put a table spoon of bleach in the brine well about twice a year. Just recently I completely cleaned the nozzle and venturi which was clogged with rust. Being city water they sometimes have a tendency to put too much chlorine in the water, It is obvious because we can smell it. We have been in a drought so I guess they have been doing it more often.

Thank you

There's more we need to know about your water conditions than just hardness.

If the nozzle was clogged with rust then you have iron in the water. You should have been following a routine resin maintenance procedure. Your resin is either fouled or mushed.

10 years on a Sears softener @ 20 gpg is a good run and far above average life. Working on those Sears type softeners is a PITA and by the time you buy resin you're close to the price of another disposable Sears softener.

Since you seem a DIYer you should consider an industry standard softener as a replacement for your Sears. A correctly sized one will outlive your Sears, be easier to work on, and should only be a touch more money.

If you want help sizing a softener then provide details about your water conditions... hardness, iron, pH, TDS, manganese, sodium, chlorine, copper, arsenic.

Also, # of people, # of bathrooms, SFR of the plumbing, and do you have any water hogging appliances like a big shower or Jacuzzi?

If you don't provide all that info then I can't help you.