I have found on many occasions this problem was caused by an obstuction in the internal trap of the toilet. Everything from toothbrushes to Batman figurines have been removed from them. If you intend to do the repair yourself you can use a closet augerwhich is a manual cable snake tool that will be inserted into the bowl of the toilet and the cable ran through the toilet's internal trap to clear obstructions. You could also rent a hand held electric auger that is basically a drill with a drum attached with a drum with coiled cable inside that is shoved down the channel the same as the aforementioned product. This device take a bit of finesse and skill as the cable will try to kink and coil up as you go one can get injured doing this. If you find that the toilet is clear of internal obstructions you may have to pull the toilet up as the obstruction may be further down the line. If you rent a medium sized electric auger you can locate the vent stack for the toilet and run your cable down the vent from the roof so you may not have to pull the toilet. This method will only clear obstructions behind the vent connection and not the length from the connection of the vent to the toilet itself. If yo have an elevated house you may have access to a clean out that by most codes are at every change of direction greter or equal to ninety degrees. You also should test your water pressure if all else fails as that specific model must have at least 25 psi at the very least to operate correctly. Hope this helps you out.