2004 Chevy Tahoe wheel is hard to turn to the right

Sometimes my steering wheel will not turn to the right after I back out of driveway and try to strighten the wheel out then it goes away. It happens usually after it sits all night but it has happend after I drove to the store and gone to back out of the parking lot. Dealer did a power steering fluid flush and no change. It does not do it every day. What else could it be?