Stubborn Incompetence

I used to be a big believer in potential. I would wait for ages for people to develop. I would train, mentor, help and teach. It took a very long time for me to accept that one does not cast pearl before swine and that some training is akin to cultivating bad soil. 

This was a rude wake up call; realising that some people are stubbornly incompetent. You will know them by their inability to progress within a reasonable time frame. (Reasonable being a variable depending on circumstances.)

You train and mentor them yet they keep making the same mistakes. They consistently give you less productivity than you paid for. You feel stressed and on edge because you can't take your eyes off whatever task you assign to them. They don't grow. 

When this happens, as a last resort, you may do something. Let someone else manage them. Sometimes, the teaching might be sound but the teacher may be unsuited to the student. So, let someone else try. If they still do not progress then reassign or sack them.