If the others are growing slower, they're using nutes slower and probably have smaller root systems that haven't accessed the whole area that the nutes are being added to, which means they might get a delayed reaction. Flushing is just throwing lots of water at it to get the salts to flush away, some people use 1 1/2 times the water they usually would, some use up to 5x the volume of the pot. No matter how you do it, you want to see run-off from that water and quite a lot of it.
Also, cut back on your feeding, my advice is to start with like 1/4 dosage applied every second watering and slowly increase through the plant's life.