Problems with Yamaha C7

[pjr]

On a well-used 10 year old C7,  the right string(only) on most of the 
high treble notes does not sound (just a thud). String sounds good when 
plucked.  Thought it was hammer alignment, but even with the una-corda 
depressed, there was not much of a difference. Filed a few hammers flat 
across the top, but no improvement.  Then noticed a indentation sideways 
on the strikepoint of most of the affected hammers.  The felt seems to 
sink in (soft) at the strikepoint,  too deep to file off.  I doped the 
hammers with some keytop solution and the sound of that string got 
better;  but this is only a temporary solution.  As I see this piano 
every ten days,  I know I'm going to face this problem again very soon.  
Has anyone a suggestions on these Yamaha hammers?

Phil Ryan
Miami Beach