You never lubed it? No wonder it won't fire.
It sounds like the nails are not entering into the slot with the firing mechnism properly (don't know the correct terminology). Most likely youv'e got debris in there... accumulated dust, sand grains, etc, or possibly rust. It doesn't take much. When properly lubed those particles get ejected with the normal firing of the gun.
Clean the tip of the gun thoroughly. Use a toothbrush, blown air to clean it. If it looks even slightly rusty you can put a few drops of vinegar all along the tip and then rinse it out after 10-15 minutes (the vinegar will eat away at any rust). Rinse first with some water, then clean it all out with WD-40 (to displace the water).