Remember that they have learnt by muffing up dozens of cases, and still are probably either playing safe by replacing many OK parts at your expense, or taking risks, again at your expense, to wind up the job fast. It needs your eyes AND mind to be working to FULL ability. Well, Bullet repair is also like that. There's NO two ways about it. Don't jump to conclusions .
Many symptoms have more than one possible cause.
Make it a habit to recall all possibilities, narrow down on basis of symptoms observed, think of the diagnostic tests that would help to narrow down further, think of the sequence which would would involve least effort, cost and time. And only then roll up your sleeves. Or even take the bike to a mechanic. It will be educative. If he won't discuss the matter with you, you need a different mechanic.