The drain in a toiler exits in a downward direction and bends sharply upward in an upside down U shape before dropping into the closet bend. This trap is what keeps water in the toilet until the volume from flushing purges it and caused the flush to occur by siphon action. You probably have, dare I say, pickles in the top of the trap. If you cant dislodge them with the plunger, using a snake with an augar (like a spring) can help to grab the obstruction and retrieve it, or push it through. Its better to retrive the object to avoid plugging farther down the pipe. A snake will be less costly than a service call.