Here’s something I’m often asked on my Sheffield piano tuning ventures, particularly when a piano has moved from one place to another and the movers haven’t shown due caution, or if a piano has not been tuned/serviced in many years. Dampers don’t damp. Notes ring out when the sustain pedal is lifted. This issue has many causes and several solutions:

1. The action is not seated properly in the piano. If every key sustains even when the sustain pedal is off, then this is probably the case (commonly occurring after a move). It takes 10 seconds to fix this, assuming nothing is broken and the action is simply loose or unscrewed. Overdamped pianos have a middle screw that allows one to move the whole action closer to the strings (this can be used prudently in order to improve the effectiveness of the dampers, although even altering it by less than a mm will have an effect on the pianos touch).
2. The damper felt is worn or missing. Depending on the severity I might suggest a full replacement of damper felt – however, a quicker solution on a piano with more pressing concerns could be to move the damper so that a new section of the felt is now touching the string (giving it new life). I have many different sized felts in stock for uprights, grands, spinets and console pianos.
3. Damper springs have lost tension – if it’s only one or two notes ringing out this can be a common issue on an older piano. In this case I replace the springs (I have a full set of upright and spinet upright damper springs in stock). I’ve never been asked for a full replacement of damper springs as a piano in such a state would likely not be worth such extensive repair. Occasionally springs can be re-tensioned manually by pulling them, but I usually err on the side of replacement.
4. On Grand Pianos only: the damper wire is stuck in the damper guide rail. Bushings can be compressed, replaced or lubricated with PTFE depending on the severity.
5. On Grand Pianos only: The damper lever or damper wire block needs repair or adjustment.
6. The damper spoons are out of regulation. This is the least common reason for a damper not working properly, as typically damper soon regulation is a task usually related to touch or how the key ‘feels’ rather than the effectiveness of the damper. In  my piano tuner’s tool case I carry a special tool for regulating the damper spoons which is often a preliminary step in a piano regulation procedure.

On many of my travels tuning and repairing pianos is Sheffield and elsewhere, I may have encountered other reasons for dampers not working, but these are the most common that come to mind.