First, make sure that your wires and breaker are big enough.
Second, (and most likely) make sure that you have energy efficient 2.5 GPM (gallon per minute) or less shower heads and aerators on all of your sinks and showers. Only use one sink/shower at a time.
Third, wrap insulation around the pipes to prevent heat loss.
It's possible that your water heater thermostat is malfunctioning too. It's also possible that one of your breakers has failed or that your breaker box is overloaded. The troubleshooting guide is very good in your manual.
I've had a JC1 tankless electric water heater for a couple years and it's more than paid for itself and I've had plenty of hot water. I've been looking for this company too to buy another heater but aparrently an american company patented their idea and thay can no longer sell in the us. Similar heaters cost between $600-800.