Q. Why doesn't my piano hold tune for very long (significantly less than six months)?

A. Mechanically,  a piano's tuning stability is primarily (but not solely) reliant on the pin block's (wrest plank) ability to tightly 'grip' the tuning pins to keep the tension on the strings.  Should the pin block be damaged or aged so that it will not sufficiently 'grip' the tuning pins, then the tension of a string (or many strings) can not be held tight by the block and that note (or notes) will slip flat in less than the 6 months expectation.
In some instances, a pin block treatment can be applied and buy anywhere from months to years of extended service. When it works, I have found it to be very effective. When it doesn't work, the piano owner has lost up to $ 300.
Pin block treatment is not a cure, but a patch. A grand piano with a worn out pinblock needs to have the pin block replaced and be restrung.  That is the time for any soundboard and bridge work to be done so the total package can be between       $ 3,500-5,500. Although theoretically possible, it is cost prohibitive to replace the pinblock on a vertical or upright piano. These pianos can be restrung with oversize tuning  pins into the existing block. There is a risk to the owner, however, that the block may be further damaged with the restringing and will still not hold tune after an expensive procedure.