The pushing with the nail opens the injector, so if you were holding the nail on it it would continue to spray. Just pushing the nail in once breaks the gum sticking it shut - that is often enough to revive it, unless it sticks again. If it ever sat more than a week or two with water contaminated gas a steel spring inside can rust and the injector will no longer want to shut back off. However, you can usually tell if the injector is just stiff or if it has no return 'spring' to it at all. In your case the two failed injectors could be something else. The injectors fire in pairs, #1 & #3, #2 & #4. If yours are not firing in such groupings it could be some electrical fault. A quick check would be to try the wire from a working injector, or check with a multimeter (but the brief 3 volt pulse could be hard to spot). If one set isn't working it is likely the distributor contacts (in the base of the distributor). There are two sets of contacts for each injector circuit.