Hi mike
If you haven't got an air lock it could be the valves inside the pump are smashed, this happened to my pump by tiny little stands of plastic,even though i had a in line filter fitted.
Give Williamson's pumps a ring they are very helpful, they replaced my valves for around £16+vat
You can suck the water through and the pump should work OK, not nice but at least you can use the pump until you fix it.
Hope this helps
John