Did you check to make sure the impeller was firmly seated on the shaft of pump? Take the housing apart and push the impeller up the shaft.

I refuse to Johnson brand pumps after the impeller on my bilge pump fell off while I was sleeping on my boat and was woken up to water coming over the transom. (I was tubing earlier that day and the rope caught the boarding ladder pulling a screw out, if you are wondering why so much water got in the boat.) The pump sounded like it was running but when I took the housing apart the impeller had fallen off the shaft. It's a very poor design by Johnson. I switched to Rule after that.