Toliet flapper doesn't reseat properly on flush
If the problem keeps occurring after a new flapper has been installed then you should be sure that:

the chain from the flapper to the handle is not too short as to not allow the flapper to seat fully.

If the flapper continues to sit askew it may be installed wrong. make sure that the two openings on the flapper arms are sitting evenly on the overflow pipes attachment points. It should not bind.

Some flappers have a hollow bulbous cavity that sometimes get air trapped inside. This may prevent the flapper from closing properly. Remove air from cavity.

Some tank cleaners that contain chlorine or other chemicals will degrade the rubber flapper making it brittle and deformed. There are ones resistant to these cleaners.