"And let us consider one another in order to stir up love and good works, not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day approaching" (Hebrews 10:24-25).
It has been my experience that skipping services is a symptom of deeper problems. Without addressing the cause, the symptom will only get worse. When attendance patterns change for the worse, it usually indicates that the person is feeling guilty over some personal sin. Coming among brethren is too strong of a reminder of their own personal failings, so they avoid the pain by not coming to services.
Coming to service is more than worshipping God, though that alone should be significant enough to get people to leave their homes. Our gatherings are also times of encouragement and learning so that our lives outside of the church services are improved. That a person doesn't desire encouragement is a strong warning of personal problems.
The only way to find out what is wrong is to sit down with the person and listen. You are not looking for the endless excuses as to why they couldn't make it last week; you should be listening for hints regarding the sin in their life. Often a person is self-deceived. They can say they weren't feeling well, yet manage to drag themselves to work the next day. Those are the excuses you want to skip. But usually buried in the conversation will be indications of something that is bothering them: personal sin, a loss of faith, or anger at a perceived offense by a brother. Don't get caught in the destraction of excuses, but focus in on why they aren't coming despite the potential hinderances.
Brethren often have a hard time handling a lack of attendance because it is a passive sin. A person says they will try to make it next week, but then something always comes up. It is much like trying to pick someone up who is totally limp. Because they don't try on their part, you don't know what you need to do on your part. Especially when there are no elders to watch, it is easy to forget that so-and-so has been to worship in months. (A form of "out of sight, out of mind.") By the time someone notices, it is often too late. Sin has piled up on top of sin to the point where even the person involved no longer remembers why they stopped coming.
By focusing on learning the cause of non-attendance, you can usually find a problem that can be addressed. If the underlying problem is corrected quickly, the attendance will generally return.