I had this exact same problem and the spring sits behind the fibre washerto push it forward and into contact with the bulb centre. The electric feed wire should have a small soldered cap at its end which goes through the washer and prevents the bulb from pushing the contact back out through the washer. The washer loses its elasticity over time and the hole gets too big to hold the wire cap in place against the bulb.
My solution was to loop through the hole several times a cotton thread. After half a dozen loops of cotton are taken through it gets sufficiently restriced to hold the wire cap in place.
