If your hoses are properly connected, aligned, and submerged it is a priming issue.
A tip to EASY prime a rena is to turn it off, and unlatch one of the latches. Then lift up on the motor housing on the unlatched corner. It will release the air in the canister, creating a siphon. This is an easy way to eliminate the air from the top of the canister. I get ZERO cavation after starting my canister up after filter maintainence!
Two things to remember:
1. There must be some water in the intake line for this to work.
2. If you do the unlatch trick, as the water fills up, and pushed the air out of the canister, water can and WILL leak out when the last bit of air is gone! It will be ok, just be fast!