first check to make sure that your valves are turned on all the way. make sure there are no kinks in the flex lines (if you used flexs). and some heaters come with “heat trapper” nipples (the short pipes in the top of the heater). that type of nipple can only go in one way, so if they are on the wrong side or upside-down. they will slow or even stop the flow of water.
hope that helps,