Often parents differ in their rules and expectations for their child. Mom might say, "You can't watch TV until your homework is finished"; but when she's away, Dad may say, "Go ahead and watch TV if you want to." Dad might insist that the child's bedtime is 8:30; Mom may say that stretching it until 9:00 is fine.
Similar conflicts can develop over issues like approaches to discipline or a child's choice of friends. When these inconsistencies occur, one parent inevitably undermines the authority of the other.
To begin to resolve this problem, you and your spouse need to be explicit with each other about what your rules and expectations are. If necessary, write them down, review them and be sure they are workable. In areas in which you differ, find a compromise that you both can live with - and stick by it.