Your toilet seems sluggish. It once flushed vigorously, but now the water seems to move slowly through its cycle, often rising high in the bowl before passing through the trap. You also notice large bubbles rising out of the trap during the flush. Sometimes the bowl even seems to double flush.
These are classic symptoms of a partially blocked trap. An obstruction has made its way to the top of the trap and lodged in the opening. In many cases, enough of the trap remains open to keep the toilet working for a time. To clear a blockage, start with a toilet plunger, forcing the cup forward and pulling it back with equal pressure. If a plunger doesn't clear the clog, try a closet auger. If the closet auger fails, bail out the bowl with a paper cup or other small container, and place a pocket mirror, bouncing light to the top of the trap. The mirror should allow you to see the obstruction. When you know what and where it is, you should be able to pull it into the bowl by using a piece of wire.