Regulated/Keydip changed

[Bigeartb@AOL.COM]

List:
Six months ago I leveled the keys, set the key height to 11/16", keydip 3/8" 
and regulated a Kimball Studio 1970's model piano. Today ..six months later I 
had to shim the front rail approx 1/8 of inch order for the key dip to be 
again set a 1/8"deep.
The keyframe was secured to the key bed  but over the years according to the 
screw heads, tuners have been removing and tightening these screws. The 
problem must be an old one. It appears the keybed has dropped in the middle 
section. Any thoughts on the cause and how to fix?

Thank you
Tommy Black
Decatur, Ala.