perhaps you created a trap in the feeder line that has air in it.
15 seconds might not be enough time to get started. The pump can run longer without damage.
You can add a nipple to that input port where the plug is - say up 18" or more and fill it with water - see if that helps to prime it.
Check for suction leaks in your new work.
How far, how much higher is the new pump location?