Shower head does not work actually I am getting no water to shower head I have 3 knobs hot cold and shower or tub when I turn the knob to turn on shower water shuts of to tub like it is supposed to but no water comes out of the head?? Rob

My first suggestion would be for you to remove the shower head and look for an obstruction in the shower head itself. Shower heads have a restrictor in them to make them comply with the 2 1/2 gal per minute regulation. These are very easy to get clogged.
Watch the way the shower head comes apart if you have a shower massager or something like it.