[Edward]

I checked my nitrates a few days ago and they had shot up to a breath taking 20 ppm.

Don't worry, 20 ppm of NO3 won't kill you. Not a problem.

My question is, why aren't the nitrates being taken up? What could the tank be deficient in to not allow nitrate uptake?

Plants need only Ca, Mg, NO3, PO4, K, CO2 and TE. If all is present then that is the maximum you can do. Plants will take only so much and that's it. If your fish produce more NO3 then the plants can take, then you are out of balance.

Edward