weekend staff

I am looking for a solution to the scheduling of staff who works only weekends (their request) My problem with this schedule comes when they wants time off. Other staff work every 3rd weekend and when they want their weekend off they must trade it with another nurse(per policy). The "all weekend" staff have been able to just request their time off with out arranging their own trade and thus others have then been scheduled extra weekends to cover this - of course causing problems. If you have any policy or suggestions it would be of help!!!