I did renovation of my bathroom sometime ago and was facing the similar problem. And I think the problem is in electrical wiring of the heater. Sometimes we are unable to find out the minute problem. So, You should once consult a reliable company like Home Quality Remodeling https://homequalityremodeling.com. Because they will know the exact problem and can fix it properly. Apart from remodeling they also provide the plumbing and electrical services so they can easily find out the minute problem.