Extra DI unit

[o.c.d.]

I had a problem with my 4 month old R/O DI unit, first I couldn't get the water below 20ppm, then the sediment and carbon chambers were over tightened or crap and I replaced them,I also replace my sediment and carbon every 3 months,still 20ppm,So thinking the DI may be the problem I ordered a replace that came with replacement DI,Instead of changing out the first unit I just added it to the outflow.Now I have 0ppm,But the last chamber will not fill passed halfway is this a problem or will this cause a problem in the future?