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?????