Being a Believer in Jesus, a follower of Jesus, involves me in a WAY of doing things, a WAY of valuing relationships, a WAY of Life.
In Ephesians 4, Paul wrote prophetically about a time of maturity in the Body when every joint is supplying what it should to the Body, and therefore we are built up to the full stature of Jesus – an appropriate Bride for the Wonderful Son. A joint is a place where two parts come together and therefore can do something.
Failing to be assembled, as referenced by the writer to Hebrew Believers, means just being present in the box, not looking at all like a new bicycle or train track or doll house. Just showing up for all the meetings is not being assembled together. Becoming a fun or useful item is.
If I am not in new relationships, or deeper relationships, or more effective-in-ministry relationships than I was 2 years ago, I am failing to be assembled with other Believers.