Kathy,
Are the actual keys sticking or is it that the hammers are sluggish and do
not return to their resting place after stricking the strings?
If the problem is in the hammers, then it would take some lubrication of the
hammers where they are pinned into the flange that attaches them to the
action rail.  The pin goes through a hole in the flange that has a felt
bushing in ti.  The felt, because of the humidity, swells and tightens
against the pin which functions as a hinge for the hammer therefore slowing
the movement of the hammers. Usually, in a climate with high humidity, you
need to install a damp chaser (a long tube with a heating element inside it)
to keep the humidity dried out.
I doubt this has been much help but I tried!  (smile)

Ned