older ders can get "unwound" loosing spring tension or sometimes the spring gives out. try to see if you can wind the tension back in..if it feels like you're forcing it you probably are so stop. good luck
Serotta Fierte and a 1989 Centurian Prestige (plus, various others)
There should be a coil spring inside the cap part that is highlighted in the photo below. Generally it's wound up and the end goes through a hole that keeps it taught. You can wind it back the way it should be using a pair of needle nose pliers. WEAR EYE PROTECTION if it's in there loose.