The problem has been described more than once with this model and evidently affects more than the odd example: the small brass mechanism inside binds at one point, and the refill can only be retracted with considerable force, or not at all. Repeated repairs through the manufacturer's service department have brought no lasting cure in such cases either, which suggests a design weakness in the range, though that has never been conclusively established.
Before any intervention, one simple check is worthwhile: if the pen has been dropped, the opening at the end of the barrel may be distorted and blocking the movement of the refill — from the outside there is often nothing to see. If the barrel is undamaged, a very small, precisely placed amount of good-quality watchmaker's oil at the tight spot sometimes helps. Too much oil does harm, because it collects grit and creeps into the refill. If neither works, the only remedy left is to replace the complete mechanism, which on a model in this price bracket rarely makes economic sense. Push mechanisms are rare at Montblanc in any case; the great majority of its ballpoints use a twist mechanism.