You either are leaking air or you have too much air in the canister making impossible to get the system to pump water. (or the impeller is not working, this possibility is not discussed)
In most canister filters the impeller is at the top of the canister, with too much air in the canister the impeller will just spin, overheat and eventually die. I am not familiar with the sun brand, but in the end all the canisters depend on a few things to work correctly.
1. They depend on a siphon from the intake to the canister, (any air sucked in will weaken or stop the siphon).
2. They depend on a constantly rising pipe from the canister back to the tank. This means the siphon on the intake side has somewhere to go. if the hose on the outfeed side has a drop below the canister outfeed then you can get an air pocket that is hard for the pump to move and the pump will not work as well. It can also prevent the proper starting of the filter if the air pocket is large enough.
3. Priming pump if present usually starts the siphon on the intake side with the hope of displacing the water and air on the outlet side.
4. The intake side should be lower in the tank than the output side.
If the pump is not pumping any water the issue is likely an air pocket at the impeller, this is often caused if the pump is not primed correctly and or the outlet hose drops below the outlet on the canister.
If the pump is continuously pumping out air with the water the most likely scenario is that air is being introduced somewhere on the intake side.
When starting the canister, it will pump air when starting because the foam, rings, .... all trap some air and this eventually works through and should be mostly quiet after a few minutes, if the siphon is working correctly then it will help push this air up and out of the canister.
The best configuration for the hoses is in some sort of V with the canister at the bottom of the V, and the intake and output at the two tops to the V. The hoses should never have a "trap" in them (up, down, up).
One thing that has happen to me a couple of times, fluval canister, is that I forget to open the valves to the hoses after maintaining the canister. The locking mechanism for the hoses will close the hose valves automatically when you remove the hoses but when you re-attach them the valve stays closed.