Use CLR to clean stuck bleeder valve
A property piped boiler system would include a dirt separator which helps to reduce the amount of dissolved solids in the system. Inhibitors can be used to improve water quality as well. Softened water is only going to help so much as a boiler system does not consume water, it circulates it. Sometimes controlling water quality is difficult and auto air vents have a shortened life due to build up on the needle valve seat. In those cases I use a ball valve like

this one pictured



Home Depot

This allows you to either periodically allow the auto air vent to function as an auto air vent or as a more convenient manual vent. It also provides a simple method for isolation to replace it in the case of a failure. It will also facilitate cleaning the air vents with CLR, which is totally fine.