Water from cistern is not washing all of the basin only the back?

I have the same problem in a new build house. Builder is trying to correct it. I think it is an obstruction in the flush pipe but as it is under warranty I cannot take it apart. Builder insists he did not drop any grout or screws down at installation but no way to check without taking the problem on myself and invalidating warranty. He thinks it is a faulty pan so is seeing if manufacturer will replace it. Just to be clear, ours flushes water more strongly from one side than the other meaning the water in the bowl swirls and does not have the gusto to clear the waste.

Tom
November 2009