I don't think pruning will necessarily help. I don't have that particular hydrangea but a related one and it seems to droop more if hard pruned as opposed to lightly because hard pruning seems to result in longer branches weighed down by heavier flower heads. It may have more to do with its natural habit, described as "cascading" in the link below from Missouri Botanical Garden, combined with growing conditions. If it is in shade, watered a lot and fertilized it may be more likely to flop. Would you be able to post a picture of it? Hopefully someome else here has that hydrangea and can comment.
http://www.missouribotanicalga...