The key might be a problem, but you could also have gotten a bad impeller. I once got one for my genset. It was pumping some water
when It was first put in but in less than an hour of run time it stopped. I opened up the pump and it looked just fine, so I started looking for blockages, etc. I was beating my head
against the wall trying to figure it out, so I finally took the pump apart again trying to figure out what was going on. I spun the shaft and noticed that the vanes were not turning at the same RPM
as the shaft. The bond between the rubber part of the impeller and the metal hub had failed. Thankfully I had another one, put it in and all was fine. You would think that for $50 for a little piece of rubber and metal they could get it right, but not always.