Check the water pressure in the boiler loop, if it's low, it can make it easier for air to get in (and to overheat in the boiler, but it doesn't sound like that is happening). Any little air in the system can create an air lock, and prevent the small circulator pump from being able to push the water up there. Depending on how the radiators or convectors are plumbed, it can sometimes be a real pain to get all of the air out. THen, if there is a zone valve, make sure that the zone valve is actually opening. Most of them have a manual override...try that just in case the valve is staying shut. Sometimes, you need more pressure from the fill connection to push the air out (sort of like after you drain your plumbing lines, then turn the water back on). If you do that, make sure you don't leave the pressure too high..., so opening the fill, and a drain line may help push air out...the pumps work fine with no air in there since the water falling back down, helps to pull the water up in the closed loop, meaning the pump really has very little work to do (and is why they can be so small). But, when there's air in there at the top someplace, the pump is too small to push it all the way up there and push out the air, that's why opening the fill valve can help. Note, you need some flow to make this work, which is why you also need to open a drain valve as well to flush things, otherwise, it's like blowing up a balloon, you'd just compress the air, not eliminate it.
