excessive water or not enough. Most cultivars are very temperamental. I hear hybridizers have come up w/ more agreeable varieties, but I have given up on them. I have some August Beauty and some Veichtii which are not too demanding but even then, they make me work for those exquisitely fragrant flowers.
ck that your soil is not sopping wet. Most people tend to overwater at the first sign of bud drop or yellow leaves. Also, try to remember to fertilize (at least occasionally). they respond well to this attention. Going into winter, no need to fertilize, assuming they're outdoors. Yellow leaves may be a sign of iron deficiency, but I don't know if lack of iron affects buds as well.