You've been diligent with the tuning of your piano, making sure it's done every 6 months. But even after it's tuned something doesn't sound right, there's something "off" about the tone. Maybe the piano sounds too bright, and if this is the case the felt hammers have hardened. Thankfully there's something that can be done about this problem, and it's called voicing. Hammers can be restored to their optimal state through "needling" and this is done as needed across the entire piano. It is a lengthy process and is done in multiple steps, but once completed your piano will sound rich and full again. Hammers don't always harden though, and sometimes pianos will actually start to sound weaker, often the result of the hammers softening. The felt hammers can be re-lacquered, restoring the tone and volume. If you think the tone of your piano isn't right, it's time to ask about voicing!