Boiler is hot, but my radiators are hardly working?

Long story short, have an enormous boiler downstairs in this old farm house - that is for the ground level of the house only - is a seperate, smaller one that is for the upstairs of the house - 2 seperate rentals. Upstairs, they can hardly breath, they have so much heat. I, on the other hand, have the huge boiler and have little to no heat to show for it. Just got my first gas bill = $550+ for one month - is insane! I have no idea what I can do to make things work and have found my way here. ANY help would be greatly appreciated. Have already had to put up plastic on the inside & outside of all my windows and still am COLDDDD! Please help... the radiators are made by weil-mclain - not sure about the boiler, would assume it is made by the same company as the radiators??