Why don't I see water being pumped outside when my sump pump runs?
If the float is triggering it to start the pump, but there is insufficient water to where the pump sucks air via a vortex, it will not pump out. That then means your float is not adjusted properly; it should not trigger on until there is enough water to avoid it sucking in air. 

Sometimes when there is heavy inflow and a pump that that was too small, people lower the float switch trip point in order to get the pump started earlier if it was having trouble keeping up with the flow. That's not the way to fix that problem, the fix is to get a bigger pump.