My resin doesn't fill all the time, either, but I've noticed it's pressure/temp related. In this cold weather, the filters aren't as efficient. If the water is warmer, the DI
canister fills with water. Same with the pressure measured on the built in gauge. Anything over 60PSI and it's fine; even a hair less, and it doesn't fill.
That being said, I can still make 3 gallons in a little over an hour. I was thinking of getting a booster pump to even out and regulate the input pressure, but I'm holding off on that.