The problem you now have mate is that the bad behaviour has now become a habit, which you obviously have to break, I would not allow the dog in question into the pick up until she does her business, you need to take her out on her own before evening exercise, I know it sounds long winded but wait with her until she does her business be that 40mins or 4, each time she does it lots of fuss as if you would do with a pup, and then load into the pick up with the other dogs, you will want this to become her new habit which will get quicker and quicker.
Hopefully that makes sense, routine is the