After motoring for 15 minutes to get the engine up to operating temperature, feel the hose in and out of your potable water heater and the bypass hose. I suspect that the bypass will be hot and the water heater hose will be cold. Maybe you have built up enough scale in the water heater to increase the pressure drop so that not enough engine jacket water is flowing through it. Either clean the heater or block the bypass.
If all of the hoses are cold, then I suspect that an air lock has developed. Open the coolant
fill cap and run the engine for a while. It will often clear up. Worst case pull off the return hose from the heater to the engine, block the inlet to the engine, and put the hose in the fill opening while running until you get good flow.
It is extremely doubtful that your jacket water circulating pump is worn to the point that it won't build up enough pressure. But the only way to know is to pull it off and look at it.