My 2001 isuzu rodeo is losing water from the radiator, no sign of leaking. over heating (of course), water from resevior is not feeding into radiator at all like suppose to.

The reservoir is used to catch overflow coolant after the coolant heats up and the cooling system builds pressure. Since you said there are no visable signs of a leak, if your coolant doesn't have a correct mixture of water/coolant or if you're just using water (I've seen this done many times) it will vaporize. Ethylene glycol has a high boiling point water doesn't. If you have a 50/50 mix you're good and I would suggest that you may have either a blown head or intake gasket and your coolant may be either seeping into engine or blowing through exhaust system. First check your dipstick, if your oil is really black change it, if it's muddy looking it has water in it. If not start your vehicle let it warm up for a few minutes and check your exhaust for any moisture dripping from tail pipes and you would smell a distinct antifreeze smell too. I would also first open the drain valve at the bottom of my radiator and run water through your radiator neck and run your vehicle until the coolant is flushed out and only clear water is coming out (be sure to note the condition of your coolant, if there's a lot of **** or rust in the system, your radiator cores may be stopped up and not allowing flow to pass through them) then replace with 50/50 coolant mixture.