Jerry Peck
I forgot to mention the obvious, and may be the easiest: is the water level in the bowl as high as it used to be when you did not have a problem?
The water level in the bowl should be as full as it will get, beyond which it just overflows the top of the built-in trap.
Try this, mark the level of the water on the bowl with a pencil, then gently press the flush handle allowing water to flow into the bowl without causing a flush. The water in the bowl will rise, then settle to the height of the top of the trap. Is the water higher than the pencil mark? If it is, the fill tube into the overflow may be blocked, restricted, have fallen off, etc.
