Some of the common reasons for clogged toilets are:
Hard Water: It contains high mineral contents that can calcify and form a strong white substance that is difficult to remove. This mineral build-up restricts the flow of waste through your system.
Foreign Objects: Flushing foreign objects—feminine hygiene products, cotton balls, baby wipes, etc., that do not dissolve in water can create blockages.
Worn Pipes: Due to corrosion, temperature change, common wear & tear, etc., your pipes may break and slow down your toilet.