You are 100% correct. When a customer brings a Glock into our shop & says it won't work, I ask:
1) What ammo are you using, if a handload, let me see some of it.
2) What did you change on the gun to make it "better"?
In about 95% of the cases, the recoil spring assy has been changed out. We start by putting the stock one back in, this cures most of the problems.
I point out that the ONLY Glock that I have that doesn't have the stock, as delivered, recoil assy. is my son's open class USPSA race gun, shooting 9mm Major power factor loads. This one did need spring tuning, as it is a 6 1/2" barrel with a custom designed comp on it. The spring had to be adjusted for bullet weight & velocity. This was NOT a haphazzard jump, but was researched based on the bullet weight & velocity required. And it was based on many years of open class shooting info.
With a Glock, stock rocks!