In any case, one of the challenges of training outfielders is there's just no substitute for experience. You can short toss by hand to work on the catching techniques, but judging a fly ball off a bat is a skill unto itself. It's not something you can really drill, per se. You just have to do it enough to get the feel for it. Some players never do get it.
There is one trick you can try if your outfielders are close, but tend to let the ball get just over their heads. Tell them to go back farther than they think they should. It sounds simple -- almost too simple -- but it definitely works.
It all has to do with consistency and making adjustments. Players who are consistently allowing the ball to go just over their heads haven't quite calibrated their brains to judge the exact trajectory of an incoming ball. They think they're under it, but instead they're just ahead of where it will land. Having them move a little further back than they think they should helps them make the adjustment, and starts to train their brains on where they should be rather than where they think they should be.
That, and a few thousand fly balls hit from the plate to the outfield, ought to do it!