There could be many contributing factors.
Listed are some of the things you may want to check:
1. Not sure what the head pressure of the return pump is but if it is too low, it may not have the "power" to pump the water up into your tank.
2. The number of bends in the return line could also cause the pressure loss.
3. More loss through the chiller.
BTW, I am not sure if a 3000l/hr pump is "powerful" enough for your tank. Well, I guess it depends on what you are keeping.
Just my 2cents while waiting for coffee to brew.
Have a nice day.