One thing I've seen before in a young single adult ward is that some sisters are still listed in the Young Women organization, which prevents them from showing on the expected lists or serving in certain callings. Because a young single adult ward doesn't usually have a Young Women organization, all sisters in the ward should be moved to Relief Society. Could that be what you're seeing?
Another thing to check is whether the membership record for those sisters is in your ward, or if you only have an out-of-unit membership record for them. If all you have is an out-of-unit membership record, they won't be able to be assigned for ministering in your ward (the ward where their full records are would have them on their ministering lists). This is probably more common for young single adults living with their parents, who may switch back and forth between the YSA ward and family ward.
Samuel Bradshaw • If you desire to serve God, you are called to the work.
