Maybe try either the next size up cover or playing with how you fasten them. Either looser or tighter could help. Tighter seems counterintuitive and there's no guarantee it'll help but it is possible that if a cover is too loose it increases movement, hence increases friction on the skin. Same with the diaper itself, play with the snap settings and maybe even folded or not folded.
Another thought is maybe think about detergent build-up or detergent sensitivity. Sometimes it could be happening just at that spot just because that's where there's the most rubbing or general contact. Maybe giving an extra rinse if you don't already, or think about switching detergents. Worth a shot, especially if you've recently switched.