Pressure Very Low Through Softener System

When do the resins in the softener tank need to be changed?

1. The average water softener will not need it's resins replaced in it's life time ( 20 + years ). However, chlorinated "city" water is known to break down the resins in as little as 5 years

2. Water softening resins need to be replaced if the unit has developed a high level of bacteria that does not respond to a cleaning with chlorine. Odors from units left in a not working mode with no water flowing through them for months at a time can develop this problem. Also, units in direct sunlight ( Florida ) can develop a layer of algae that requires complete resin replacement.

3. An excessive amount of sand in resin tank, due to a well starting to collapse, can necessitate resin replacement.
For more Info:
Check out the Resin Replacement Guide

Can the softener cause pressure loss, if so what do I look for, and what do I need to fix it? 
Yes, a softener can cause pressure loss in the home due to resistance from the resin bed caused by one of the following.
1. On well water, this is usually due to fine sand coming from the well.
 2. On softeners installed in the open sunlight ( mostly in Florida ), a layer of algae can grow and thick pieces of this growth clog the lower distributor tube screen when they start peeling off the inside of the resin tank.
 3. On chlorinated water supplies, sand can get into the tank from new construction or work on water lines in the area. All of these situations are rare. 
4. The most common cause of pressure loss occurs on chlorinated water.
The resins can be damaged by high chlorine levels and turn to mush. This has the same effect as having fine sand at the bottom of the resin tank. 

The solution for all of the above problems is to dump the resin tank, clean and rebed with new resins. One cubic foot of high capacity ( 8 % cross link ) softening resins is enough to properly fill the average residential softener. We can calculate the amount for you, if you provide exact resin tank dimensions. And we have common tank sizes and capacity chart on our Resin Replacement Guide web page
The second most common reason for pressure loss occurs with Autotrol control valves. 
The flapper valve discs swell up when used on chlorinated water. This causes the holes in the valve to become block, resulting in pressure loss at high flow rates. To see an example go here
The solution for this problem is to replace the valve discs set and the backwash flow control ball ( this part also swells up ). See Autotrol Parts Page