I am a software engineer and I work on a team of 3. A colleague of mine is a very qualified software engineer.
However, he does not focus on the most important project (as defined by our manager). While I am actively working on the project our team should be working on, he often finds ways to do other work that he like more.
We had a big migration planned today, and two days ago while we needed to prepare it I asked him if he could do a small task (15 minutes max) and he just told me "No, I am working on something else".
I clearly saw on his screen that he was working on adding an extra feature asked by the user on a previous project of ours that we are not supposed to work on anymore without planning.
So he just refused to help me on our top priority, to do a zero priority task
I insisted, and he told me "Hmm, okay I will do it". The next morning when I arrived I saw that he did not even did it.
I was very upset at him, but I can't find a way to tell him that he need to focus more on the task at end or I will report his behaviour to the manager. I fear it would look like I am threatening him or patronizing him.
Is this a good idea to talk to this about it? Should I talk about this with my manager? Or should I just keep it to myself and don't say anything to anybody.