It's likely that you have an air leak and that zone is blocked by air. The boiler water pressure, especially on the first floor when the upper ones are working, probably isn't an issue. The pressure is more to keep the water from boiling, but it does help keep air from infiltrating. Normally its somewhere 12-16# or so.
If your expansion tank is nearly shot or its precharge pressure is not right, in between cycles when things cool off, any potential air leak could be sucking in air. these may not leak water, since when hotter, things expand. Look at all of the joints for evidence of (minor) water leaks. If you find any, they'll suck air when things cool.
You may also need to service or replace any air extraction device you may have on your piping. Normally, air migrates to the top floor, or high point in the system, but could stop at the first floor depending on how it is plumbed.
If the system is leaking water, and the system has an autofill valve, the new water will have some air disolved in it. You need to find the leaks, as this constant source of air and oxygen can promote corrosion in the system.