The Senco SFN-1 gun will drive one nail and then not retract

First get an exploded view of gun and parts list and numbers from senco.com,,take nail gun apart,,,take a good look at the rubber bumper stop to see if it is broken,,look at the driver nut to see if it is tight,,,,you can go to juldan.com once you have gotten a list of the worn parts,,,juldan sells complete o ring kits along with nearly all parts needed for your repair

Some reasons the driver/piston assembly won't retract fully:
- damage to the o-ring (LB1003) around the piston
- cylinder (BA0165) is scored badly
- driver (EA0167) is bent, twisted or warped
- o-ring (LB1023) around bottom of cylinder (in groves & covering small holes) is stretched out or out of place.
- Seal (BC0492) at bottom of cylinder & under bumper is damaged or gone

First, check that you are using the proper nails. The FP602 needs Full Round Head nails with plastic collation. If you are using Clipped Head nails with the paper tape collation the angle of the nails not let the driver catch the next nail and drive it into your work.
If you have the correct nails, check the magazine to make sure the nails are feeding all the way up to the driver. If there is anything blocking the way or if the spring on the feeder isn't taught enough to pull the clip all the way up the driver won't be able to catch the next nail and drive it in.

You have damaged o-rings in the gun. Remove the head cap & valve assembly, remove the ring on top of the cylinder, remove the cylinder & piston. The cylinder is in tight so work it out carefully. Check the condition of the following: a split flat ring with a rubber o-ring under it on the top of the piston and two o-rings on the outside of the cylinder. If these are damaged air will leak around the piston instead of pushing it back up. The inside of the gun should be clean with a light coat of oil on everything if you are oiling it properly before use. Dirt in the gun means the gun is being laid on the ground without the air hose attached. Dry innards means no or not enough oil is being put in the gun before use. Oil pooled in the bottom of the gun means too much oil is being used. Senco doesn't make a rebuild kit for this gun, you'll have to go to the Senco website http://www.senco.com/ or other website like http://www.ereplacementparts.com to get a parts breakdown of your gun and order the o-rings (any part beginning with LB) individually. If you replace one, replace all of them and grease them up liberally when reassembling the gun.

oil real well

Not sure about Senco but most have latch you can open to expose firing mechanism. Nail sometimes gets jammed and will not allow driver to fire or if not nail push driver ****** all the way back into gun then see if gun will fire.

The Senco FP600 is an older model gun, if it has been getting reguar use check the driver. After a lot of nails the tips tend to get rounded off towards the back side and if the nail hits some dense wood or a knot the head will slip off before it can be driven home. Replace the driver if so, trying to grind the end square will only result in problems with the guide when the driver retracts into the head.

you need a top end rebuild kit

firing pin called the driver is probably broken or needs new seals max air pressure 125 psi senco sells repair kits

hi visit your supplier and ask for a overhaul kit once fitted your tool will be licke new
best of look ian