My water heater has a recirculating pump that has failed. I replaced it with pump from a big box store which is intended to be installed at the top of the water heater, with a cross-over to be installed between the hot & cold water shut-off valves under a sink at the far end of the house. I did not install the cross-over valve, and installe the pump where the old one was at the base of the water heater. I am not getting hot water at the distant fixtures, no matter how long I let the pump run. Why would this be? Is it possible the new pump isn't strong enough? Is it possible the check valve is stuck and keeps the recirculation from occurring. Any suggestions you have will be greatly appreciated, Thank you.