Is there oil in the injection pump? I would imagine if the oil in the IP was not replenished for a long time, or leaked out, or if the car had been sitting a LONG time, the plungers inside might start to seize up & seals dry up. Somewhere on here or another forum I read about someone with a diesel fintail (200D) reviving their "frozen" pump after it had dried out, I think by taking it apart & soaking the plungers in penetrating oil. I don't think I would dare to do that myself though as I have the habit of having leftover parts after I put everything back together
Another problem could be air (from a fuel line leak) that has become trapped in the IP....there is an air bleed screw on it.