I am a team lead with 5+ developers. I have a developer (let's call him A) who is a good programmer, who writes good clean, easy to understand code. However he is somewhat difficult to manage, and sometimes I wonder whether he is really under-performing or not.

1. Our company requires the developers to indicate the work progress in the bug tracker we use, not so much as to monitor the programmers but to let the stackholders know the progress. The thing is, A only updates a task progress when it is done ( maybe 3 weeks after it is first worked on) and this leaves everyone wondering what is going on in the middle of the development week. He wouldn't change his habit despite repeated probing. ( It's OK, developers hate paperwork, I do, too)
2. Recent 2-3 months he on leave quite often due to various events-- either he is sick, or have to attend a lot of personal events etc. ( It's OK, bad things happen in a string. It's just a coincidence)
3. We define sprints, or roadmaps for each month. And in the beginning of the sprint, we will discuss the amount of work each of the developers have to do in a sprint and the developers get to set the amount of time they need for each task. He usually won't be able to complete all of them. (It's OK, the developers are regularly missing deadlines not due to their fault).

If only one or two of the above events happen, I won't feel that A is under-performing, but they all happen together. So I have the feeling that A is under-performing and maybe-- God forbid--- slacking off.

This is just a feeling based on my years of experience as programmer. But I could be wrong.

It is notoriously hard to measure the work of a programmer, given that not all two tasks are alike, and there lacks a standard objective to measure the commitment of a programmer to your company. It is downright impossible to tell whether the programmer is doing his job or slacking off. All you can do, is to trust them-- yeah, trusting and giving them autonomy is the best way for programmers to work, I know that, so don't start a lecture on why you need to trust your programmers, thank you every much-- but if they abuse your trust, can you know?

My question is, how can you tell whether your programmers are under-performing? Surely there are experience team leads who know better than me on this?

Outcome:

I've a straight talk with him regarding my perception on his performance. He was indignant when I suggested that I had the feeling that he wasn't performing at his best level. He felt that this was a completely unfair feeling. I then replied that this was my feeling and I didn't know whether my feeling was right or not. He would have none of this and ended the discussion immediately.

Before he left he said that he "would try to give more to the company" in a very cold tone. I was taken aback by his reaction. I am sure that I offended him in some ways. Not too sure whether that was the right thing to do for me to be so frank with him, though.

Extra notes:

1. I hate micromanaging. So all that we have for our software process is Sprint ( where tasks get prioritized and assigned, and at the end of the month, a review of the amount of work done). Developers would require to update the tasks as they go along everyday.
2. There is no standup meeting, or anything of the sort. Mainly because we have the freedom to work from home and everyone cherishes this freedom.
3. Although I am the one who sets the deadline, but the developers will provide the estimate for each tasks and I will decide-- based on the estimate-- the tasks that go into a particular sprint. If they can't finish the tasks at the end of the sprint, I will push them to the next. So theoretically one can just do only 1 or 2 tasks during the whole sprint and then push the remaining 99 tasks to the next sprint and still he will be fine as long as justifies this-- in the form of daily work progress updates