could be a combination of difference in ground moisture, temperature, etc. A lot of mine have been very slow opening this year. It happens from time to time. Have patience. As long as the buds are still on the bush and green. They will open. Nothing's wrong, they are just taking Nature's Time.
PB

I have the same exact problem with my gardenia right now. I know a gardenia isn't a camellia but maybe the cause is the same. The buds seem to just keep getting larger but won't open. I guess I shouldn't complain. I almost fell over when I spotted the buds almost TWO MONTHS ago! Patience, you said??? LOL!

One Bush is three years old. When I bought it had lots of buds and they opened. Last year a few opened. This year none. The other bush, I just planted last year, way across the yard from the other. It is loaded with buds that don't open.