It takes them a while to fill out. If you want very upright plants, prune hard (you can go all the way down to a single stem, or just a couple of strong canes) late winter, or at the beginning of the season. If you prune now, all you'll be doing is cutting off blossoms. Another tactic to take is to let nature do its thing, and not prune, and the plant will even out on its own, as it gets bigger (don't tell proven winners, but limelight loves to root those little stems at the bottom of the plant that everybody cuts off - I'm all for free plants). More blossoms this way, but the panicles won't be as large.

Only prune from Fall to Early Spring. Limelight blooms on new wood. Just like posted above, pruning during growing season only results in cutting off blossoms. Also if you hard prune you will get larger flowers!