Charlie, I notice nobody has posted, probably because we don't know the exact answer, so I'll take a shot. It may need fertilizer when it is not growing or blooming vigorously. Other things (not enough direct sunlight) will cause the same response, so you'll have to look at your situation to know for sure. I'd simply suggest maybe using a water-soluble fertilizer once-a-month or so as you water as a good starting point.

Here is a link that might be useful: Garden Musings blog

In addition to what Professor Roush has posted above, another indicator would be leaf color: are the leaves a deep green color or are they a pale yellow green? If they are a pale yellow green, it could indicate a nitrogen deficiency, one of the 3 "macronutrients" needed in larger quantities than the "micronutrients". If you google around, you may be able to find a series of color photos that illustrate the symptoms of each of the macronutrient and micronutrient deficiencies. Roses tend to be heavy feeders, so if you have not been feeding them regularly, chances are they need to be fertilized. I don't have much experience growing roses in pots, but I feed my full size roses monthly with around a third of a cup of Vigoro Rose Food (endorsed by the American Rose Society-has all the needed macro- and micronutrients) once a month from March to September. If I had a potted rose, I would use a smaller amount.