Swimming Pool Pump Priming Issues
According to the manual:


  Note: The pump is only self-priming
  when filled with water. Refilling is
  only necessary if the pump has
  been drained, or if all the water
  has been lost.


So, the "self-priming" label just means you don't have to fill the entire inlet hose. (Seems a bit misleading to me, but such is life.)

Edit: since this pump is designed to be a permanent part of a pool, just pouring some water into the pump the few times it's allowed to go dry isn't a big deal. It's a bigger problem for your application where (presumably) the pump is drained and put away between uses, but that isn't what the pump is designed for, so that's probably as good as you'll do with this pump.