Scripture says to "forsake not...assembling", but it doesn't say how often, with whom, with how many, etc. Sooooo...I think it is the motives of our heart and our reasons for missing that will factor into whether its a sin or not. I've known folks to be out of church every service for months because someone close to them was dying of cancer and needed constant care...but they were still faithful saints of God!
Staying away from church because you hold disdain for pastors and saints would be a poor reason...and indicative of heart issues that need to be resolved.
Staying away from church because you want to play video games instead of going to worship and being taught God's Word smacks of selfishness.
Staying away to spend time with family seems like an okay reason, but one must wonder...why not take your family to church and worship together?
Staying away because of illness...is understandable.
Staying away to rest is understandable...sometimes.
I suppose it depends on the reasons. And the overall picture of faithfulness...or lack thereof. And really, only God and the church-service-misser know for sure.
When a person suddenly begins to skip church, it can be a symptom of bigger issues, which sometimes (but not always) include sin.
Or it may be a symptom of nothing more than a worn out family provider working a 60 hour week and maintaining his home and family on the weekends and evenings. Or a mother of toddlers having a hard time balancing
anything in her schedule, including church.
There are good reasons and bad reasons to miss church. You can't just say across the board that missing church is a sin...unless you just want to be hard to get along with.