Check for a shutoff valve between floors.

And check for a corroded return line or valve that's not letting water circulate

lazygardens - Would the potentially rusty/corroded valves be located at the base of the radiator, or are you referring to something that would be in the basement attached to the boiler or water tank? Thank you!

You must keep bleeding them until you get steady water discharging for several seconds. Then if you still don't have heat, check the circulator pump(s) (if any). Then I would move on to such things as the auto-fill valve or have the pipes checked for blockages. Which may get expensive.
But start by paying closer attention to bleeding out the air thoroughly.
Casey

I suspect Brickeyee is right. If the top of the highest radiator is higher than 20 ft above the boiler, the pressure probably needs to be raised above the optimum pressure. But have a plumber show you how to do it the first just to be sure you're doing it properly, then you can do it whenever it is necessary. I have to do it every year.

Jlc, there are some real experts on old radiators at the Heating Help forum who might be able to advise, and they also offer maintenance-oriented books & manuals that have helped me a lot:

Here is a link that might be useful: Heating Help Forum