The problem with trying new pellets is some will need a leaded barrel to group properly, whereas some will prefer a clean barrel, I always choose a pellet that groups well in a clean barrel as it's easier to maintain the accuracy.
Does the rifle now not group with a pellet it grouped well with previously ?? if so what has changed ?
If the rifle has had a full service it might also be worth checking the alignment of the pellet probe as they are renowned for pushing the pellet sideways into the rifling which ruins the grouping