You have a Girard GSWH-2 heater. Reading other reviews, there is a lot of discussion about low pressure and flow causing this not to work properly.
For the shower, don't mix cold with hot water. Run it full hot and see if there is enough demand to trigger it. If not, you need to change your head to one that allows at least 2.5GPM of flow.
You can also try upping your pressure, as it needs a minimum of 8PSI according to specs.