Sounds like you have the right membrane for well water.
This has me stumped.
Unless there's a tear or damaged RO membrane - it "should" remove the nitrates.
When you're done using it - like running a batch of purified water - do you keep the blue "good water" tube elevated above the membrane ?
Doing so will keep the water from draining out - and insures that the membrane is always kept wet inside.
If the membrane dries out - I believe it can become ineffective.
I'd suggest contacting Kent Marine directly -and talking to one of their RO techno-weenie guru's.
I've never added a deionizer to an existing RO unit - but I'm sure it's an option. One that you could discuss with Kent.
Kent Marine