Answer: Since your site is in full sun and the plant is otherwise healthy, I suspect the plants are missing essential nutrients. Roses benefit from applications of greensand, which contains potassium, and bone meal, which contains phosphorus. Both of these nutrients are important for flowering. Even micronutrients in granular form would be helpful. The thing to avoid is feeding them with high-nitrogen fertilizer that will encourage foliage growth at the expense of flowers.
I hope these suggestions make the difference!
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