If the refill still extends but no longer springs back into the body when pressed again, the cause usually lies not in faulty reassembly but in the compression spring. It tires over the years, or gets squashed and bent while the pen is being dismantled; it is then too short to trip the catch reliably. The cure is to draw the spring out carefully to its original length – after that the click mechanism will as a rule work dependably again.
When reassembling, the order of the parts and the alignment of the cam track are decisive; fitted the wrong way round in relation to its guide, the mechanism produces exactly the same symptom. Since the individual parts are small and apt to ping away, it pays to work over a shallow tray. Force is never needed: if something will not go, a part is in the wrong position. Should the spring be broken or permanently deformed, replacements are available from dealers in writing-instrument spares; the manufacturer's service department will also replace small parts of this kind.