There are a couple of possibilities. The most common issue is some kind of suction side air leak. That could be a bad seal on the pump strainer basket lid o-ring, or low water level letting air into the skimmer. Or it could be something more serious, like a leak in the plumbing.
Another possibility is that it is trying to draw water from too many sources. If you have more than one source of water that you can control with valves, try closing all of the input valves but one and see if it will prime from there. If that doesn't work, try isolating each water source in turn to see if any of them work.