If it's an old toilet that is intalled and stop siphoning it may only be obstructed. If it's a brand new toilet that you've just installed, make sure your drain pipe have a good angle, because if it's nivelled you won't have draining pressure.
It could be as simply as that the bowl is clogged with large stools or a simple object accidently dropped in the john. Also, your septic field may be too old.