yellow knockout only blooms half way up
I have a yellow knockout that is several years old and has always performed as a knockout should. However, this year, it has buds and blooms half way up a bush that is about 7 feet tall and the top is green and healthy but no buds or blooms. I fertilized it as I did all my roses with 14-4-14 and none of them are doing this. I have around 600 roses but this has me baffled. Can anybody help me?