Flints need a good sharp edge to produce a lot of spark. That's why the old timers carried a little bitty hammer so they could chip them along the edge. So the flints may be dull. or, they could be set just a little too far back. set them a little closer and snap the hammer and see if you get more spark. If not, it is probably the edge. You can buy new flints for just a couple of bucks from:
http://www.buffaloarms.com/Muzzleloa....aspx?CAT=4786