Has it always put out the same amount? If so then it is probably your pressure and or cold water. I have pressure that runs at a low of 35 and high of 55 and my water is well and very cold so I hooked up a Y hose on my washer outlets and run some hot and cold together. This helped my out put, and no, the warm water did not do anything to the final out put.
I was burning the DI
resin faster this way so I got a 8800 booster pump. WOW what a difference that made 80 PSI now., I crank out 5 gallons an hour and I have been using the same resin and filters sense May. Still producing 1 on the TDS
I would first look into a pressure gauge and see what your really running.