I see two avenues being taken. The first is a person having problems getting up to speed. I had some issues with that too, albeit all headwork mistakes on the practical exams (yes, with a "s").
It can be overcome. As soon as I identified my issue, I was able to push past that, and have never failed another ride. After that point, I haven't failed again. As a matter of fact, I think I've done pretty well from that point on, ending up as a checkpilot in 121 operations.
On what SafetyEngineer said, I agree, as it relates to a person employed at a 135/121 operation. If you have been trained, current an qualified at an airline, and have a pattern of unsatisfactory performance, there is a serious issue. Everyone has a bad day. When it's a trend that one continually is unsatisfactory in an aircraft they have a significant amount of experience in, there is a serious issue.
I don't think, though, either scenario can be inter-related, unless via a specific individual. The required types of learning are different. One is testing the individual on new knowledge. The other is verify the quality of knowledge and skills already learned, experienced and ingrained.