This is a hard question to answer. Roses often have a number of insects and disease issues that cause wilted brown buds, so it is difficult to give you a definitive diagnosis. Some of the possibilities are insects or disease.
Thrips can do this, but are rarely a big problem in home gardens. They are minute (one-sixteenth of an inch) insects that attack rose buds in their early stages. Rose blossoms turn brown and buds open only partially. The brown edges of the petals appear to stick together. Remove and destroy all infested blooms and buds.
Botrytis blight, Botrytis cinerea, causes the buds to turn brown and decay instead of opening. Buds may also have a gray mold. The fungus is most active when temperatures are 62°F to 72°F and conditions are moist. Avoid problems with good garden sanitation. Pick up leaves and fallen blossoms so the fungus will not have a chance to produce large quantities of spores. Keep the foliage as dry as possible. If the plant is super dense, remove some canes to allow good airflow around the plant.
Some rose gardeners use fungicides, to deal with diseases, but most people don't have the time or inclination to do this.