Recently Ive been adding a lot of death certs to my tree, but not all of the people that are dead after a certain period have them available. When are death certs released to the public? Are they held for a certain period of time like census records?

For example, I found a death cert for a 2x great grandfather, but not my great grand mother. They both lived in the same place, and I have enough info on my g-gmother for ancestry (or any other genealogical sites) to be able to find a record should one exist in their system.