Why does my hydrangea form buds which turn brown and never bloom?

My blue hydrangea gets lots of flowers but only a few petals open not like the full blossoms I had when first planted. What happened?

I suspect that the buds and flowers turn brown because they are being infected with the disease botrytis (grey mold). This disease can attact other plant parts as well as other plants. It is found just about any where plants grow. It releases spores on rising and a lowering humidity, needs free standing water (from rain, over head watering or a dew) to infect. It infects at temperatures between 32 and 84 degrees and over winters on both dead and live plant parts. You can try treating with a fungicide till flowers open to minimize the amount of infection. Also remove and destroy all infected plant tissue.