You may be running into a lack of nitrogen--roses are heavy feeders of nitrogen. The woody bedding sometime found in horse manure locks up nitrogen, making the problem worse.

Here is a link that might be useful: Washington State Study of Horse Manure

I have had a similar experience. I only fertilized once a year when I first started growing roses and had quite the welcome change in my bushes when I started using compost from a local farmer. I threw it out there twice because I had so much compost to use up and kowabunga! What blooms I had!

Roses are greedy feeders! Organics are always good but a little bit of NPK can do wonders!

I think that you may be using Vigoro Rose Fertlizer. This is a product that has been endorsed by the American Rose Society and has been designed specifically for use on roses. Besides its "macronutrient" formulation of 12-6-10 (12% nitrogen, 6% phosphorus, and 10% potassium), it also contains all the essential nicronutrients that are usually missing in most granulated fertilizers. I have been using this product for well over a decade, and the results are spectacular.