First have the heater checked to make sure that the DIP TUBE isn’t broken, which can provide only the top water for a short while. Also have the heat source checked to see if it is functioning properly. Also, make sure there is a flow limiter to 2.25 gallons in the shower head. If after it is checked and proven that all is good there, it is a small heater, less than 30 gallons, the amount of hot water can be increased by adding a water heater or installing a new one.
http://www.heatpro.us FREE heat/cool right-sizing
Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.