(MOVED FROM PLUMBING)
The radiator in the spare bedroom only heats up at the bottom and not the top, tried bleeding it and no water came out after aout 20 minutes, so put the screw back in, not sure if its connected? I was told there may be air in the system, What's the solution? Thank you, Stacey

You should make sure pressure on boiler is between 1 and to bar or it could be sluge in system

The boiler water pressure is probably to low. On a normal system it will read aprox 12 PSI on the altitude/pressure gauge. Most units have a auto fill valve but yours may not. Do not overfill if manual and you will have to feed more water in as you bleed if a manual setup.