When the motor is cool, pop off the radiator cap and start it up. Let it run with the cap off till it gets up to tempature and the thermostat opens and the water is flowing good in the radiator. Let it run and let any gases / air out of the system that may be trapped (shouldnt take but a few seconds once the t-stat opens). Top off the water level and watch your water in the radiator. There should be no more bubbles at this point.

You may have filled the radiator with water but not the motor. Be sure the T-stat opens and the level is full before you put back on the cap. HTH