If you see water running thru your pump but not the return, you're filling your filter with water. Turn on your pump, open the valve at the top of your filter and let all the air hiss out. When it spits water, close the valve. You should then see water going out the return into your pool.
If you don't see water running thru your pump, you're losing prime - you've probably got an air leak before the pump. Make sure the secondary strainer lid o-ring is in good shape, clean and lubricated. Make sure the lid is screwed down tight.
Let us know if this doesn't work.