we have baseboard heating with two zones. two baseboards in part of zone 1 are not heating. they happen to be in a separate room furthest from the thermostat but previously put off at least some heat. I bled the lines from both of them and was able to get hot water to flow through after about 20 - 30 min. but it did not last. we are going through a cold spell but not sure if that is the problem or if there is something else contributing?

so much depends on how the baseboards are piped. i'm guessing you have a split loop system.
if you do then one side could heat and one side could be frozen.
as I said, without knowing how they are piped, everything is just a wild guess.

If you were bleeding them correctly, it should NOT take 20-30 minutes for hot water to flow through the zone.

what do you mean by "bleed correct"? one at a time, I opened the bleed valves on each baseboard that wasn't working and let the water flow for 20 - 30 min on each. I did turn up the heat to get the boiler going to be able to feel if temp was changing.

If things were working properly, all you had to do was open the vent until ALL the air was gone and water STARTED to flow. That is what I meant by "properly". Venting for 20-30 minutes has NOTHING to do with bleeding the system. IF there is no air in the system and it does NOT heat, almost immediately, there is some other circulation problem, but one we cannot diagnose without checking it out, since we do not know anything about your system.

The thing about baseboards vs. the old fashioned stand-up radiators, is, there is no top for the air to collect in. It is just a pipe, so when you open the bleeder, you may get all water flowing from one side of the baseboard, even though the air is on the other side. I don't know that air is the problem, but you need to be able to control the flow when bleeding, by shutting a valve on the return line.

The "top" in a baseboard is the top of the pipe, since air RISES to the highest level, and an air vent on top of a tee at one end WILL evacuate the air from the top of the pipe, regardless of which direction the water is coming from.

You are assuming that all the air has made it to the baseboard, which you can't. The water will take the path of least resistance, and that may not be where the air is.In that case, you could bleed for days, and not get the air out.

The air will move with the water flow, unless it is trapped in an "inverted U", in which case there will not be any flow), and accumulate at the high points in the baseboard.