What is your water hardness? To my knowledge, like Buckeye Hydro, your vendor does not plumb membranes in series with no knowledge of the feedwater quality - we leave that to another not-so-well informed vendor.
Is the check valve after your permeate tubes join installed backward? Look for an arrow molded into the body to indicated correct direction of water flow.
Are your DI carts 100% tightly packed full with resin? Is the change to orange you're seeing at the top or at the bottom of the DI cartridge?
Russ