Now there could be many reasons why this is happening. 1 the diverter may be assembled incorrectly. 2 the diverter/cartridge may be defective. 3 the plumber may have used to much solder, filling the pipe thus blocking the flow. 4 There is a shut off valve somewhere that's closed.
Do you have a home warranty? If so I would suggest calling the builder and inform them of the problem. Any respectable builder gives a warranty, I believe in most states its mandatory. This should be covered. Just a heads up, this could involve some holes being opened in the walls, ceilings, and floors in more than one place.