DI resins must be flushed just like prefilters, carbons and membranes. DI is shipped in a pickling solution that needs to be flushed away, usually several bed or canister volumes will do it.
Did you check your RO only TDS to see if its the DI that needed replacing and not the membrane? Test your tap water TDS and RO only TDS to determine your rejection rate, its possible the DI is pulling double duty for a worn membrane.
The DI canister not filling up is normal, the water travels up through the center of the cartridge so what you are seeing is normal. Hold your DI outlet higher and you will see the canister fill up more, then lower the line and you will see it fall, no big deal. If it bothers you, when the unit is running slightly unscrew the canister to bleed the air out and it will fill up but will probably drop again once it shuts off.