I'm guessing that there's air in there somewhere (my new song lyric!).

There's a cheating way to assure that you get all of it out that we used on R5 Renault's (The Le Car here in the US). Get a spare radiator cap and cut the middle out of it. Get about a 1 foot length of appropriately sized tubing (we used 1 1/2" EMT conduit). Braze or solder that to the radiator cap such that the tube sticks up. Get a good sized steel funnel and braze or solder that to the top of the tube such that you have a reservour to hold the coolant (a trick here is to add the antifreeze to the lower radiator/block first and add pure water with the rig in place - that way any spills are just water). Anyhow, start the car and do the typical burp the radiator routine with the rig in place.

This rig assures that you have a head on the coolant so that any bubble has a higher place to which to escape. With the R5 it was a real PITA but they did have an air bleed so you could release air from a pocket (seems like it was the heater core) as you did the ritual.

These days there are pressurized systems available (my buddy has one at his professional shop) that force the air out - his uses a slight amount of compressed air. But the system described does work and costs little.

Dan