Not enough hot water to complete shower. Noticed

If it is a new water heater it might be that the water supplies are crossed. The cold water should be installed on the right hand side of the tank or on the bottom if your tank hooks up on the side. If its an older water heater the dip tube may have broken off. This is a tube that runs from the top of your water heater on the cold side to the lower portion of the tank.It puts the cold water in the bottom of the heater which pushes the hot water to the top.To change the dip tube out you must disconnect the cold water supply from the 3/4 inch nipple at the top of the tank. Then with a pipe wrench unscrew the nipple from the tank it will have the remainder of the tube on it.You can get a replacement dip tube at a plumming supply. Installation is opposite of removal. If this is your problem I suggest you get a new water heater .Usually its caused from minnerals in the water or buy two dissimiler metals being connected together in the water piping system causing corrosion Your water heater will probably fail in the near future.