In many pianos, even new ones, the keys are tight and bind on the
balance rail pins.  Test for this by simply lifting the front of the
keys up a couple of inches; they should drop back down with the weight
of the action.  If they don't, ease the keys with a special tool made
for the purpose; an oval-shaped tapered pin that you slide into the
balance pin hole on the underside of the key to compress the wood
fibers and expand the hole.  Make sure to insert the tool so it expands
the hole side-to-side, NOT front-to-back, which would make the key
unstable.

Other possible binding points are the felt key bushings at the top of
the balance rail pins or under the key at the front pins.  These are
eased with special pliers that compress the felt and wood.  It takes
some observation, practice, and experience to diagnose where the keys
are binding and take the proper corrective measures.

Easing keys in this way can make a dramatic difference in the
performance and feel of a piano action.

Larry Lobel
Virtuoso Piano Service
Petaluma, California