Steam Radiator problem

I have been trying to figure out why a steam radiator at the end of the line is not working even when the riser is getting hot on the radiator itself.  I have taken out the vent and it still does not heat up.  It seems to be taking all the heat to the radiator about 5 feet away but does not get to the one at the end of the line.

Should I try and put a vent at the end of the line so the steam to get there quicker?

Not sure what the problem is, pelase help.

Thanks

Rob