If the plumber supplied the toilet (you bought it from him), he should make it work, otherwise, you'd pay for a return call. Several things: the chain could have too much slack, and it is getting caught underneath the flapper valve. Or, it could be too short, and is holding it up (or has a kink in it, not letting the flapper seat). Or, there's a defect on the seat where the flapper sits (or just some crud stuck there - it should be smooth and even). Or, the flapper is bad (Toto will send you a new one if you call). A close visual inspection and maybe a feel around the valve seat should identify the problem.