How long did the pump and lines sit around?
I am assumeing that the single injector getting fuel is one of the 6 operated by the injection pump and not the cold start valve(which operates all by itself from feed pressure).
I can't think of anything other than plugged injection lines or 5 frozen delivery elements that could cause a problem like that. If the pump sat with the lines off for months(or years)rust can form in the bores of the delivery elements causing the little pistons to get stuck the first time they move through the rusty spots. Verify that the lines are clear then remove the screw-in pieces from the top of the injection pump and get some penetrating oil on the elements in the pump.
If that doesn't work, its rebuild time for the injection pump.
ASE Master Tech
Mercedes Tech 19 Years