I sometimes feel that the nitrite or nitrate sticks around in the substrate so that when I do a massive water change I am not really removing that much of the nitrite or nitrate.
I have often done something close to 90% water changes, yet not seen a change in the test result.
When I do a water change that includes draining the water through the substrate, then the test result shows that the nitrite or nitrate has dropped. Dig a small hole where it won't disturb the plants and siphon the water out through there. This will do something closer to 100% water change, and a lot of the water will flow through the substrate, hopefully removing the nitrite.
At the same time quit feeding the ammonia for a few days. The bacteria will not starve. It will give the Nitrospira (the nitrite removing species) a chance to catch up. Then start in with the ammonia again, but just a little bit.
Often the first group of bacteria, the ones that remove ammonia grow so fast that they just keep on producing the nitrite so fast that the nitrite removing bacteria cannot keep up.