I bought a replacement radiator once, aluminum core with plastic ends. Turned out the hole for the tube that goes to the overflow was never drilled out.
I would suggest checking for an obstruction either in the hose or in that tube. Could be a simple fix.
Also check the hose that goes from the overflow tank cap to the bottom of that tank. It can be gunked up at the bottom. Also check the vent tube of the overflow tank.