Something we have seen happen when systems suddenly cannot build pressure is debris clogging the check valves. There is a non-invasive solution that may be able to remedy this without disassembly.
Please reference the below linked video, specifically the procedure at the 1:48 mark.
If you kink the brine discharge to stop flow when the system is running under pressure, you will see the feed water PSI rise quickly. It should cut out at 125 PSI from the pressure switch. Repeat no more than 10 times in succession to 'shock' the system and hopefully reset any check valves stuck in the open position.