Like Art said, the rim of bowls have small ports around the upper inside, and these can get plugged up with calcium deposits and whatnot denying the toilet the ability to properly flush and often exhibiting the same symptoms as if the toilet was plugged up normally. Easiest fix is to take something thin, strong and pointy, like a coat hanger, and carefully poke it through all these holes around the rim. Give it a try and see what happens.