Do not use WD40 on your rifle!!
As for short stroking, clean off all that WD40 first. Rifle grease was meant for hot, wet conditions, oil was used for most environments. I use a lot of 10W-30 Mobil 1, works great. So clean up rifle and then use an oily patch on the metal surfaces.
If the WD40 was not gumming up the action, evidenced after the removal of WD40, and improper function, then you have an insufficient gas issue. Either the gas piston is worn or the gas cylinder is too big. A situation analogous to worn piston rings in a combustion engine.
If the rifle does not cycle after being properly cleaned and oiled, call the CMP . They should have gaged the gas cylinder and piston but if they did not, they will fix things for you.
If I'm not shooting, I'm reloading.