Something came up on a job recently. After filling the rainwater tank the pump wouldn’t start pumping water. This is protection provided to stop damage to the pump if it runs out of water. If you have one of the pumps without the air cylinder on top chances are it has an automatic cut-out.
When this happens to you cycle the pump a couple of times (turn the power point on and off) until it picks up the water.
If this still doesn’t get it going you may need to call a pump specialist