I'm not an expert, but this what I had to do.
Do the 50% water change but don't redose the ammonia right away. Wait an hour then retest the water. What are the reading then? If they are still off the chart, do another water change, wait an hour, and retest. Repeat until the values are back on the chart. Remember to redose the ammonia after you get the water where you want it.
How much ammonia are you redosing to right now? 4-5ppm I should read more carefully.
When I hit this point in my cycle I backed my ammonia dosing off to 2ppm. This was my logic. Someone can correct me if I'm wrong. You know you have enough of BB1 because the ammonia is almost gone every day. You know you have some BB2 because there are Nitrates, but you need more because there are still Nitrites present Why keep adding more Nitrite, via stronger ammonia concentrations, when you don't have enough BB2 to keep up already?