Try turning the pump off and see how full the sump gets. If the sump only fills up half way when the power is off, you can add some more water to it to fill it up. Then there should be more water for the pump. If the sump fills up completely when the power is turned off then it won't help and you will need to get more water draining into it from the tank.
Sumps can cause tanks to lose a lot of water due to evaporation. A coverglass on the sump can significantly reduce water loss and help keep the water levels stabile.