If I let my old sub pump run dry I just submerge it and turn it on its side as for some strange reason it primes quicker that way. Mind it's years old so don't come with a float switch that might be the case with your particular issue. Does the pump turn on when you hold the float switch up?
Good question Smurf. Those float switches work either way depending on which wire you use. I have the MachineMart supplied one and I use mine the other way to a submersible. I have to connect my black wire to the positive feed to fill the tank. To empty the tank the blue wire is connected to the positive.
So to empty the tank the black wire isn't connected but made safe.
If the pump activates when the float is hanging down, the wires aren't connected correctly.