I would say inexperience. Even with a proven management system there are a lot of people that say they can but really just want to give it a crack and hopefully earn an easy buck.
My PFC for example (on an auto 230rwkw still going strong) first off went to my usual mechanic who is a great bloke, no bs, listens to what I have to say as generally I'll only make a request or suggestion if I actaully do have some idea. He said he'd have a go however also said he'd never done one before. All honest and up front, he didn't have much luck but sorted out a couple of other things for me and didn't charge me for the time he spent on the PFC. Next guy said no worries, does them all the time etc, ran like garbage and had to reset it within a couple of days to get it at least running properly myself. So third time lucky no more mucking about, straight to Racepace and Ben sorts it, done deal, no problems.