Give them a gentle tug to see if they have been uprooted by either weather or voles (horrid critters that eat the roots). If they are firmly in the ground, then I would suspect some winter damage. You could also dig one up to take a look at the roots. Some of my minis did something similar between years 2 and 3. Keep them watered, not much fertilizer, only something like Rose Tone, and keep an eye on them. Is there good drainage?

Kate..
What kind of horticultural care are these roses receiving? Have you checked the soil pH lately? Roses do best in soil pH around 6.5 -7.
Have you improved the soil? Make sure they aren't standing in wet soil as roses don't like to have wet feet.
Are the roses healthy or do they have fungus diseases?
Minis don't require a lot of specialized care, but a well balanced fertilizer properly applied is also a key component to their well-being.
Hope this is helpful.
Meg

Last year they received alfalfa tea with epsom salt or fish emulsion on about a two to three week rotation. They are sprayed with a fungicide and pesticide about every week. When the plants were put in the ground, I amended the soil very well. We naturally have clay or sandy soil depending on where in the yard you are. I added potting soil, composted manure and that white stuff to lighten soil. (sorry the word escapes me right now) Other roses in the same area with the same treatment have done well. They do get some spotting but I'm pretty good with the spraying. Perplexing...
thanks,
kate

Humm. My first worry would be root problems. Could you dig up one of the small puny ones and see how the roots look?

Spraying with a fungicide is a good idea, but regular spraying with an insecticide is not. You may be killing beneficial insects like bees. Only spray when you actually see something dining on the roses - or the tell tale signs. I agree with the others, dig up a small one and check the roots. If the roots look okay, then I'd cut them all back some and only keep them watered - but not overly wet. Don't fertilize until they return to health. If the roots don't look good, go to ars.org and click on "Ask Questions About Roses". There you will find a list of Consulting Rosarians from all over. Look for one near you and e-mail them. Maybe they can come over and take a look at your roses. It is difficult to diagnose the problem without seeing the roses in person. Good luck.

Here is a link that might be useful: The American Rose Society