All the way back during Season 2, shotgun slugs were extremely inaccurate in comparison to the standard pellet spread. The problems were rectified in Multiplayer when Season 3 was released but now Season 4 is in full swing, the same issues can be found in Warzone.
Reddit user "Sundancelc" experienced the issue where his VLK Rogue shotgun should have eliminated his opponent with ease but instead, the slugs seemingly disappeared into thin air. A few managed to connect with the enemy in the middle of the fight but by then, the player managed to turn around and down him.
It's widely considered that the slug ammunition for the VLK Rogue is the worst kind to use, with players often opting for the default ammunition thanks to its superior accuracy when firing from the hip. Many players often select the Dragon's Breath ammunition, ammunition that ignites the enemy when it connects to deal additional damage over time.
Until there is a fix for the issue, it's best to stay clear of the slug rounds for all shotguns.