We had a software tester in our QA team that had been reporting a lot of issues, but most of them were low-priority visual bugs, browser-specific behavior bugs, sophisticated (but with low probability of happening) bugs involving using multiple concurrent users, poor network connection, insanely fast "cybersport-style" clicking etc.
On the other hand, he was constantly missing serious defects that were sometimes noticed too late on production resulting in "expensive" fixes.
So, to recap, he was doing his job - he was testing and reporting issues, but it looked like the problem was with his testing focus - instead of focusing on critical functionality of the application under test, he was concentrating on esoteric unusual use-cases.
How would you deal with this kind of situation?
In some sense, this is a different perspective on a How should a Software Tester deal with missed Defects/bugs in Production? question.