Dropper posts will often start to feel slow or sticky after a fairly short period of time. A basic service to clean the wiper seals and lube the internals is all that’s needed to keep them feeling fresh for a while longer.
Rock Shox Reverb:
If your dropper post has stopped working entirely, or feels really rough then it’ll need a full service. This includes a full strip down of the entire post, clean, and rebuild with fresh fluid.
Cartridge type dropper posts (anything other than a Reverb):
If you cartridge dropper has stopped working then it’ll need a new cartridge unit installing.