Hopefully this is a easy question with a simple solution.

I recently bought and moved into my new home, a ranch built in 1955. It has a single-zone forced hot water heating system. Due to oil costs, I have been keeping the house relatively cool (~58 degrees), so I haven't been paying too much attention to the temperature. However, this morning my freezing girlfriend finally noticed that the spare bedroom was the only room actually being heated - the fins and pipes in the master bedroom, living room, kitchen, and bathroom were all cold to the touch. I cranked the heat up for a while and it did not improve. The pipes in the basement are hot everywhere in the run, but when I go upstairs to the corresponding ones, those same pipes are cold to the touch. The strange thing is, the spare bedroom has the heaters farthest from the boiler. The previous owner had a new oil tank installed right before we closed because the old one had a leak; I'm not sure if this would have had any effect or not.

So, any thoughts on why the hot water is passing right by all these other heaters for the farthest one in the loops? Any help is appreciated, I'm really hoping this is a DIY problem and I don't have to pay a plumber or the oil company.