Air collecting in radiator??

[Day]

We had a new central heating system fitted when we redone the whole house about 4 month ago. It included new boiler, pipes and radiators. Right from the begining there was a leak in one of the soldered joints but that got fixed. What I have been noticing since then is that the bathroom radiator which is also in the highest place is slowly collecting some air. By slowly I mean 4-5 top vertical bars of the radiator become cold over a period of 1 month. The pressure in the boiler shows as normal and it only drops if i bleed the radiator. I have not noticed any leaks and checked the connections around the radiators. It is only that one radiator that collects the air, all others don't even have an inch of air.
What can be causing this? Is this expected to happen and I just need to keep bleeding it every now and again?

 

[Day]

Ok, some more info. I did a test which i just read about. I put a lighter next to the vent when bleeding the radiator and the gas lit up. The flame was shooting like from a gas burner!
Does this mean some corrosion?
The whole system has been drained at least 10 times since it's been installed so I assume all the stuff like flux should have been washed out. The water in the when I bleed the radiators is quite dark in colour though.

 

[Burnerman]

The gas is actually hydrogen, typically produced when the system is dirty....which I agree yours shouldn't be!
However, clean it with one of the quality cleaners on the market and then refrain from draining down once inhibitor has been added. Fresh, oxygenated water is the last thing you want in your system.
John

 

[TicklyT]

Do you have any corrosion inhibitor in the system?

If not, expect corrosion, sludge, a generally poor performance and short life expectancy for your boiler and radiators.

IME it takes some time to clear all the air and gases from a freshly filled system as the water chemistry settles down, but the intervals between bleeding radiators should increase, something like the next day, then the next week, then the next month etc.

Constantly accumulating gas is usually indicative of corrosion forming in a system with inadequate inhibitor, or a poorly designed open vented system drawing in air.