Impossible to say without knowing what it's been through but a few things to try.
Keep it full at all times. The extra weight seems to help the plunger stay down.
Flux your pot, stirring down not just letting it burn on top.
With gloves and pliers, use a firm piece of wire to clean out the inside of the spout up to where it stops (where it meets the plunger). I use 174 lb SS fishing wire.