I am completing what is lacking in Christ’s afflictions for the sake of his body, that is, the church.
Wait a minute, just a bit ago Paul was writing about how Christ’s ministry is all sufficient for salvation, and now he is completing what is lacking. What gives? It is enough to drive me to the commentaries. In general the notion is Paul’s suffering is connected to, perhaps an extension of Christ’s suffering, as he proclaims the Gospel.
Andrew Lincoln writes that Paul is making use of a concept of Jewish messianic woes envisioning a time of worldwide tribulation during which suffering has to be borne by the people of God before the new age is ushered in. It’s notion of suffering fits with Paul’s, specifically in Colossians. Unfortunately, there is no place for a suffering Messiah in these Jewish woes. Nonetheless, understanding Paul’s afflictions as part of the missionary task of proclaiming the Gospel, and that such proclamation is incomplete until the parousia, leads us to hear Paul speaking of missionary work not redemptive work. 
So, Christ’s ministry of salvation is all sufficient. Relief. However, the parousia is not complete, meaning there is missionary work to be done, and all of us are called, according to our various gifts, to ‘suffer’ with Paul in his suffering with Christ; even for folks we’ve never meet.
Given the remarkable applicability of Amos’ prophecy concerning economic justice, and Paul’s demanding model of suffering proclamation, one might accuse the lectionary authors of piling on. But hey, they did not write the scriptures, and they certainly aren’t responsible for aggregate human behavior. Still, the combination give one pause, there is work to be done. As Christ’s disciples it is ours, and as Paul says, it is not convenient.
Perhaps it is symbolic this doesn’t want to come to a close. So as I ponder a sufficient lacking I bid you anon.
 Andrew Lincoln, New Interpreters Bible, The Letter to the Colossians