sluggish hammers on spinet

[Pianogreig@AOL.COM]

I've never before encountered a spinet (in this case a 1963 W****T***) with 
all hammers so sluggish that moving them toward the strings by hand results 
in many floating back to rest & many at the extreme ends just staying at the 
string.  Protek on hammer flanges showed little or no result; key bushings 
are not binding; strengthening return springs also had little effect.  Piano 
was not played or tuned for about 10 years. Any suggestions - aside from 
replacing it with a decent piano? (owner says it really has a beautiful tone, 
of course).

Is it just a combination of factors: poorly weighted keys, weak springs, 
nonuse, etc.?

Thanks for any suggestions.

Bruce