First of all I would like to thank anyone that can help.
I am on a public water system and the water pressure is fine throughout our two year old home, except in our master shower.
This is a relatively recent occurance. There doesn't apprear to be a leak. The only thing I can think of is to snake the pipe or check the opening of the valve to be sure it opens all the way when I turn the (1) handle. Can it be anything else?