My camellia flowers are partially opening - about half way, then they are withering.
Certified GKH Gardening Expert
Make sure you are keeping the soil evenly moist and that your plant is not in too sunny of a location.
A balanced fertilizer is best.
Nutrient deficiencies can cause leaf, bud and flower issues.
The symptoms you list however may be Bud Mites. They can cause bud drop and the decline of flowers.
They can't be seen with the naked eye, you may be able to find them with a magnifying glass.
Neem Oil is the best treatment for Bud Mites.
Here are some links with more information.