Advise careful thought before adding more. Something wrong with the testing, would seem the first thought but then you've shot that theory already. Heavy fish and the resulting feeding is always enough to get me lots of nitrate. I do have trouble with getting the correct readings with API but it is a situation where I get totally high readings when my nitrate goes over 25-30. I now cut my tests, diluting with distilled so that I can get a logical color to read. Your problem would seem to be the total opposite.
Reason for caution would be that while a lack of nitrate will slow the plants, a totally overload may begin to kill fish if it gets too high?
Testing to be sure the cycle is complete and not lost in some way? No ammonia, nitrite spiking that would indicate the waste is just not being processed down to nitrate yet?
Random thoughts that may not help but the best I've got at the moment.