This is going to sound like horrid advice, but often you can find out the answer by digging up a rose and looking at the roots.
Looking at the photo, I can make several guesses, but I'd rather not because there are too many possibilities.
This spring, I had one rose out of one hundred that did not leaf out along with all of my other roses and finally dug it up and found out what the problem was and learned that the rose did not have a chance of succeeding in that location no matter what I did to care for it.
My best suggestion is to dig it up and see what is happening underground.
