There are many reasons. The most common is tank adjustment/repair. Is your water level an 1" below the overflow pipe and is your flapper operating correctly. A worn flapper will close too soon. What kind and how old of a toilet is this. The older the better.

You can look inside the tank at the ceramic dating.

If you have a newer low flush toilet then some brands flush and some don't regardless of what you do. Did this happen all at once? If an older toilet is there calcium build-up at the siphon hole in the bottom of the bowl.

You may also have something in the toilet and/or your house drain is starting to clog. This site's reference library has excellent explanations of toilets that may help you further identify your problem.