Is the smaller one in part shade rather than full sun? Roses do best in full sun. Is the smaller one planted by a tree or other shrub with roots competing for the nutrients? What about water? Is the smaller one getting less water or too much water? Another thought – is the smaller one in the pathway of a dog or other animal that is packing the dirt around the soil and not in a good way. At this point in the year, I wouldn’t give it any fertilizer to encourage flower buds. Top dressing with compost will be good. Hopefully, these questions will help you figure out what’s going on and come up with a solution for next year.
Thanks for writing!