We have radiator and baseboard heating (2 zones). They are all fed by hot water as opossed to steam. In the winter (and probably other months although I never check then) I have to bleed them quite often. Maybe once a week or every other. It seems some of the radiators don't get warm nearly as quickly as the others so I'll try bleeding and sure enough there is air in all of them. I think I get it all out then a couple weeks later there is more. Do you know what is causing this and how I could possibly fix it? Thank you

When you bleed air from your baseboards it must be replaced with an equal volume of water. Does your system have an auto fill and can you confirm it works?