Carrier dog spring and/or plunger might be sticky - see Part #6 at http://www.urban-armory.com/diagrams/rem870.htm
. Pull your trigger plate assembly and give the carrier dog spring and plunger a drop of lube and see if that helps...
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.