Are you guys doing matched parties in DF or forming parties in PF?
If in DF, if you do "Purpose : Duty Completion", it's only supposed to match you with people who haven't cleared for the week. You can be matched with people who have cleared it if you choose "Purpose : Practice". If Completion matches you with people resulting in a one- or no-chest party, you should file a bug report.
I don't think the PF option for "Duty Completion" has this same restriction. Consistency would be nice.