I've been there!
I can't say how many times I've flown in to somewhere...opened up the local instruction manual, and started at Square 1...
Customers don't want to see that. They want the Factory Guy to come in with a glamorous set of tools and diagnose some bizarre malady. The best was a vibration issue at a company in MO... I was there on the machine during overhaul...and now...three years later it was vibrating. Customer diagnosed it...I was there to reassemble.
Long story short...they had sent the electric motor out for overhaul when it shorted phase-to-phase and after re installation their fears of compressor damage seemed justified in a high vibration trip that wouldn't go away.
Puzzling to me was if I rotated the compressor by hand it was fine...then, sitting over the coupling pushing down on the 48" Pipe Wrench rotating it it came to me: normally I crawl onto the machine from the high speed side (opposite of what I was) and push down... I thought "I'm rotating this backwards!" I did my scroll check, then asked they get the manual, section 5, page 4 (standard standard, standard!)
The motor company, despite marking everything for orientation, had reversed end bells of the motor. The customer did the rotation check, reversed leads, then had a vibration problem that just would NOT go away! They even said to me when I got there "it's strange, we an roll it in reverse and it's fine..."
One basic check to the manual (which clearly said: "compressor rotation is clockwise viewed from drive end"!) and they would have never torn into a perfectly good machine. Nor had to pay for a field Balance!
I flew across the country, assembled the machine, and in 45 minutes and one start diagnosed reversed end bells (improper rotation)...
Later compressors had a big cast-in direction of rotation arrow...hmmmm wonder why?
Most of the time it comes down to something very basic, but overlooked or dismissed.
The head of maintenance sat there, looking at the book....and said "we probably did all this work for nothing then..."
Yep! And balanced a field balanced rotor that made it imbalanced and needed another field balance to get it back where it was before you took it all apart!
Don't get discouraged, just step back...and then comeback with a fresh outlook and check it all again. You are right: a second set of eyes s invaluable when looking at this...it was JeffP and a case of Moosehead before he said "Man, your laptop's comm chip has some really funky signals coming out...let's try mine!"
Badaboom! Fixed...for about 20 minutes and then my power circuit went DOA! Second box was fine and that box has been in there since!