some baseboard heaters not working while others are in same zone
It sounds like the second floor system is split (one zone, but not a single loop of pipe) so that water is circulating in half (or three-quarters), and not in the other half (or quarter).

You either need to look harder (pull the covers off if you have typical fin-tube baseboard) for bleeder valves you are not finding, or cut the system open and add some. You have classic symptoms of air in the system, and there should be bleeders at the high points of the system, since you really can't get all the air out by bleeding down in the basement. If you have any bleeders on the working sections, certainly give them a try to see if there's any trapped air there, even though they are working.

The small-percentages possibility other than that is that the system has actually frozen on that side. Air-lock is far more likely.