Without knowing your TDS numbers, temperature, pressure and waste ratio there is no way to troubleshoot the system, all those numbers are critical and the only way it can be done.
Just because someone says you have the correct flow restrictor does not mean that is correct, you need to measure the waste ratio yourself as it is rarely correct with fixed restrictors.
Yes, you should be replacing the sediment and carbon block filters every 6 months and disinfecting the system at least annually if not at every filter change since it only takes 5 minutes at most and you already have the system apart for service anyway.
DI resin has a shelf life of about 6 months under perfect conditions and much less if not stored correctly, moist in a vacuum sealed bag and in the back of the refrigerator. Once the bag has been opened it dries out quickly and loses its electrical properties. Try not to buy any more than you use in 6 months time, one cartridge or refill from Spectrapure lasts me about a year so I make it a point to buy it about a month before I expect the old resin to exhaust.
If you have the 100 or 150 GPD membrane you need to pay special attention as both are less efficient than the industry standard 75 GPD and will eat DI resin much quicker. Places like Spectrapure will not sell a 100 GPD Dow Filmtec and they discontinued the 150 as it did not meet their quality control testing. Stick with a 75 GPD Dow Filmtec or Spectrapures treated and tested 90 GPD which starts out life as a Dow 75 but gets improved.
It just about impossible to own a RO/DI wihout having a handheld TDS meter too, its your only way to know what your system is doing. They are only $20-$25 for a very good one and can be a life saver.
