Sticky or Sluggish keys can come about due to a variety of reasons. A careful examination is thus necessary to perform the necessary repair to nurse the affected keys back to working condition again.
Some reasons which could result in the malfunctioning of keys could be fractured keys, cracked key buttons, loose key bushings, loose leads, oxidised leads, loose balance holes and pulley keys.
If the key doesn't move freely, its bushings may be tight or the key pins oxidised. If the resistance is greater toward the ends of the key travel, the balance hole may need to be reamed. There maybe debris or excess glue inside bushing mortises, or the key pin may be nicked. The leads may have expanded and are rubbing against the neighbouring key. The key or the key end felt may rub against its neighbour, or the backcheck may rub against the neighbouring hammer, as in the case of grand pianos.
Hence, the usage of the correct tools and substances is necessary towards the diagnosis and repair, without causing damage to the other surrounding moving parts or the wood finishings.