Answer: I don't think you're doing anything wrong. Brown-edge buds are a common indicator of several problems ranging from pests to nutrient deficiencies to environmental conditions. See if any of these might fit your particular case:
Thrips - the pest larva is the most common cause for brown-edge petals in the spring and early summer. The larva spends much of this part of its lifecycle scurrying about inside your rose buds sucking vital plant juices from succulent new growth. To determine if your rose has thrips, pull back the bloom's petals with your fingers and look for small "slivers" scrambling about looking for cover.
Controlling thrips is a bit more difficult than discovering them. Controls include high-pressure watering wands designed for insect control to natural predators like lacewings and predatory mites to an arsenal of synthetic and organic pesticides.
Balling - a condition caused by high humidity or over watering from above the rose. Some roses are more susceptible than others. Blooms only partially open and petal edges begin to turn brown and rot. To determine if balling is the cause of brown-edged petals, feel and smell the bloom. If it feels slimy and smells like something that belongs in a compost bin, balling is the most probable cause.
Controlling this problem is not easily done unless it is caused by over-the-top watering, in which case the irrigation system should be changed to water from below the plant. The plants susceptible to balling will usually overcome the problem later in the season when humidity is lower.
Heat stress - a condition caused by planting new roses (usually grown in a greenhouse) into a new, full sun condition without hardening them off before planting. New growth and buds are desiccated by the hot sun conditions and very common if the roses are planted where they receive only west sun. This is most noticeable in darker roses.
Hope this information helps you solve the problem!
Q&A Library Searching Tips