It's a little unclear what is going on.
Check the campaign overview tab to see if leads are currently in wait step: X Leads in Wait Step.
How many leads completed Campaign A?
How is Campaign B picking up leads from the List? Is it
Trigger: Added to List IS "List A" ?
Is Campaign B a batch?
It could be that leads in A have already run through B once and are just getting to the List and being ignored.
You can certainly do a special campaign:
Campaign C:
Smart List = Members of Campaign A OR Members of List A and Members NOT IN Campaign B
Flow: Remove from Campaign A; Request Campaign B
so you'll need to add a Trigger to Campaign B: On Request FROM Marketo Flow Action