Diagnosis
Coffee oils and fine grounds have built up in the lid's plunger mechanism, creating a sticky, gritty paste that jams it.
How to Fix It
Disassemble and deep-clean the carafe lid. Twist and pull the black plunger button straight up off the lid. Remove the inner plastic strainer/gasket. Soak all parts (except the outer silicone seal) in hot water with dish soap or a cafiza tablet. Use a small brush to scrub the plunger shaft and its housing.
If That Didn't Work
The internal spring may be corroded or damaged. You can sometimes find replacement springs or entire lid assemblies sold separately.
Heads Up
Clean the carafe lid every week. Rinsing isn't enough. Oils solidify and act like glue. A clean lid is smooth and requires only a thumb press.