How did you determine that coolant flows only one way? Does the radiator consistently get low, and the coolant reservoir overflows?
Check to see that the hose isn't pinched or crimped, and that it reaches the bottom of the reservoir. Also check for leaks in the cooling system. You might also try removing the radiator cap and the reservoir cap so that you can blow air from the reservoir tube to the cap. If air doesn't flow easily through the tube, there is some kind of kink or blockage. Lastly, check the for any holes in the tube going from the cap to the reservoir.