My cordless hitachi NR90gc after driving a nail the piston wont retrace to ready position i have to use a small screw driver push it back and fire again . can anyone help . thanks

Maybe there can be a problem with a rip in the cilinder its a very thin cilinder and its neccesary to replace with all the parts on it because of the extreme heat build up.
cilinder assy 42 41A possible necessary, but first inspect the cilinder carefully on a rift. some other rings or parts broken or worn.
Hitachi NR90GC 3 1 2 Gas Powered Clipped Head Framing Nailer Parts

Did You drop it?
AirNailerParts Nailer Parts

The allen screw/s that you mention are there to plug holes where automatic equipment would normally connect (such as automatic nailing machines) . This is only a plug and should not be driven in too far. I have also seen allen screws installed in the head to repair small crack/s.

Make sure that orings are in good shape, lower bumper not cracked, (no leaks and the driver/piston stays at the top position, if piston drops a little or all the way down with no nails in gun, replace piston oring). Also check spring tension for feeder and two check paws in the door (feeder tension should be firm ). Adjust the timing screw up or down one or two turns and test (timing screw at the top of safety foot, head of screw pushes on the firing valve ). Test for lower bumper cracked. Remove nails and leave side door open. Hold the nose of the gun against coiled air hose or thick rubber and press and hold the trigger. The piston will stay down against the bumper until you release the trigger. No air should leak out the driver path. If air leaks past driver suspect bad bumper. Test action of feeder piston. Remove side cover to feed piston and clean debris. With air disconnected, move the feed piston back and forward. There should be a little resistance, if it moves very easy, suspect orings worn. As soon as you connect the air line. the feed piston should move forward with force, note the position of the feeder. Now with air disconnected, place nails on the feeder. The first nail should be well within the path of the driver. If not fully under driver path suspect worn feeder / feeder retainer pin, or debris inside feed piston chamber preventing piston from full forward travel (debris from inside of the gun will enter the feed piston chamber usually broken bits of lower bumper). Replace parts as needed. Good luck with your repair.

Give it a thorough cleaning.. if it still does not fire you might have to replace the spark plug.

Yes, BUT the paslode Round drive nails and known to cause driver jamms due to the excessive paper used in the spacing of the nails.

It sounds as if the piston o ring is worn. Does the piston drop if you just give it a slight bump with no nails in the magazine, if so that is the problem and would need replaced. Let me know otherwise and I will dig deeper

The roofers that frequent my shop want their nailers to work like machine guns. Here is what I do to make the hitachi 45 nail fast. Remove the cylinder and sand the fine scratches where orings meet cylinder. I use a rubber slug with a small bolt through the center. The rubber slug fits snuggly into the cylinder and the bolt into chuck of drill. Turn the cylinder with the drill while holding fine sandpaper against the scratches untill smooth. remove the nose from body and replace the sheet (part # 877125) and the bumper. If the sheet fits loose on the driver and the bumper has cuts or missing pieces, the driver will not return with full force. also the oring on the piston should not be too tight. Just snug enough to hold the weight of the piston. While nose piece is off check the air passage to feed piston for trash, use a small wire and push through to make sure. Make sure the orings on the feed piston fit snug. Actually push the feed piston by hand in the nose piece to make sure the three orings fit snugly. A loose feed piston will reduce nailing speed. Finally replace the boot on the firing pin. If the boot is blown off its seat with use replace the firing pin. A loose firing pin will waste air and lower pressure reducing nailing speed. Good luck