Take your bolt and firing pin out again. Clean and degrease thoroughly again. Do not apply oil or grease to your firing pin. I've seen countless misfires in cold weather from oil on a pin. Roll the firing pin across a flat surface looking for straightness. Feel entire pin for burrs, mushrooming, etc. Same for the hole in your bolt breech face. While you're at it replace the firing pin spring. With the bolt and pin assembled, push firing pin in an out feeling for any resistance.

With your trigger assembly out, degrease it thoroughly with something like Gun Scrubber. While holding the trigger back (to prevent sear engagement) push hammer back and forth with other thumb feeling for any resistance, binding, or gritty feel. On a gun that I am going to be using frequently and therefore cleaning frequently I do not use any oil in the trigger or hammer assembly.

In addition to what devil dog said about the gas system, check the notch at the right rear of your chamber that the extractor fits into. See that it is free of crud, which can prevent your bolt from closing completely.

If you are shooting in cold weather I would say you probably have an excessive oil build up in these areas. In warmer weather, replace firing pin spring and look for mechanical resistance slowing your hammer or pin down.