Plumbing Questions? Ask a Plumbing Expert.
There are a couple possibilities. The toilet bowl could be partially plugged. The best way to clear that is with a specialized snake called a closet auger (see pic attached). If that doesn't work to may need to snake the main line which may require a call to a pro since a large motorized snake is required. A (partial?) main line stoppage could also be blocking a plumbing vent (or the vent by itself could be blocked) which is another possible culprit.