Eileen,
Many times this is caused by something in the trapway of the toilet. A makeup pen, shampoo bottle or toy can find it's way into the toilet and lodge inside the passageway.
Sometimes a "snake" or closet auger will dislodge the object. If not, removal may be needed to remove the object. If the toilet has always performed badly, this would be a good time to consider replacement with a better performing toilet.

Regards,
Terry

• Toilet Information (click here)