@nova honestly the more I practice Tae Kwon Do the more I'm convinced this is The Truth
at every belt graduation there's always someone who can't break their board on the first try and needs more coaching to make it happen (they always get it in the end)
and while part of the answer might be like ... align your kick better, or whatever, 90% of the time the main thing is that you just didn't yell enough! somehow this works every time!