Hammer Fails to Strike the String

My Universal Piano Company player piano has been getting weaker
when playing. It seems that every system is working well, but over
the years it started not striking a note now and then.

I am new at working on player pianos and I have had to make up
my own tools to check things with. A vacuum gage is one of them.
I am going to buy 1 of your manuals, but I'm not sure which one
to get. Please help.

I have a Universal Piano Company piano, Model P-2, Serial No: 6100.

Thank you for your help,

Rick Hyden
Aurora, Missouri