Sometimes it takes a year or two to adjust while it builds a good root structure. I wouldn't feed it much if any until maybe early fall and then treat it next early spring as a more mature and settled in plant and feed it accordingly.
I don't know if KO's are like regular roses that benefit by deadheading the spent blooms but you might try that next year after the first bloom.

I have a knockout rose hedge. What you experienced is the 1st flush of flowers -- gorgeous wasn't it? Knockout roses are "self-cleaning" -- they will eventually drop their old brown blossom heads but if you do it for them they will re-bloom quicker. I use Bayer Advanced Systemic liquid on my roses. You can apply it 4 times during the year (5 times if you have sandy soil like I do). It fertilizes, protects against bugs & disease. I try to time my applications so that when the Japanese Beetles come out the roses are already protected. Hope this info helps!

We don't do anything to our KO's except cut off the dead flowers, and trim for shape in the spring. Some bloom constantly, some bloom in spurts. Try burying (just enough dirt over it to keep fruit flies off) a banana peel near the base of your bush.