Probably air stuck in the system. Was it purged correctly? When replacing the coolant you have to get the air out by running the motor with the radiator cap off until you see it purge a bunch of air out. Once it stops shooting out air bubbles you refill it, put the cap back on, let it cool, and fill it the rest of the way or overfill your reservoir.
If that fails, probably looking at a head gasket or radiator. Or one of the parts you put in is bad. Process of elimination is all you can do.
