I can only think that there's a leak somewhere.
Try this, take the outlet pipe off the hose reel. so now you are left with just a couple of feet of hose coming from your pump. Start the pump, then fold the hose to stop the flow. If your pump stops, you know that there must be a leak further down the line. If it doesn't stop, then either you have wired the varistream up wrong, or the varistream is faulty.
As the Varistream cuts off the power to your pump when it senses the pressure build up, the pump can not be at fault. Dai