air bubbles inside the engine block itself and/or fluids not getting into the block which is part of the plumbing for the cooling system. the radiator isnt the thing thing holding the coolant.
you gotta make sure you're getting fluids pumped throughout the internals of the block and crack open the air bleed screw (if there is one, i cant remember and am out of town atm, sorry). hope that helps!
If it took only half of the usual amount to fill to the top of the radiator cap bung there is the possibility that there is a blockage somewhere, and that can be a bit difficult to find considering the complexity of the system. In general blockages happen at choke points such as the junction between the block and the head, any valves that may be in the line, heater cores especially in the sleeper, and any other feeds such as the turbo, oil / coolant heat exchanger, system air pump, etcetera.
In general such blockages can be dislodged by thoroughly backflushing the system (against the flow direction of the thermostat, that is if it has one) with a combination of clean water and maybe 1 to 5% dish soap -- just recover it to inspect the outflow.
To determine the direction of flow start at the radiator and trace the hoses to the water pump: in most systems the pump will be the first thing after the outlet of the radiator (it's much more efficient to push it through than pull it), and distributed from there. Auxiliary feeds can come off from a variety of places, making determining flow direction a bit difficult, but with the use of basic logic you should be able to figure it out.
What engine are you currently running? With more specific information I could check in with several mechanics I know for (hopefully) more ideas.
If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
------- Rob --------