This doesn't sound like a backspot problem from a coach's point of view, based on a couple of things you've said.
1. If the stunt can hit with one back spot but not the other, that might only mean the 1st one was doing extra -NOT that the 2nd wasn't doing enough. There are plenty of teams...
If there are too many negatives, then it's likely that the skill is a bit too hard for the athlete to accomplish correctly. Take them "back to the basics" so to speak until they can show "perfection before progression." You could also break the skill down into smaller parts (or drills) until...