Your axe blade might be ground at the wrong angle for splitting. Ideally, you need two axes, with blade of the "splitting" one ground to a thicker angle, so it doesn't penetrate the wood as far as a thinner blade.
For a big log, don't aim at the middle, but towards the edge nearest to you (that is safer than aiming at the edge furthest away, because of where the split pieces of wood and the axe blade are likely to end up). The wood in the middle is the strongest part of the tree, so aim at the weaker part instead.
Another "trick" is not to strike the wood exactly in the direction of the grain, but at a slight angle, so the axe blade forces the wood apart as it tries to cut into it.
All these tips work on the same basic principle - you are not trying to cut through the wood (which will fail when your axe gets stuck and comes to a stop, if you can't cut all the way through with a single stroke) but to make the wood break into two pieces along the grain.
Note, it's impossible to have an axe that is "too sharp." As with other hand-held cutting tools, sharper means safer, not more dangerous, because the axe is less likely to bounce off the wood and hit your legs or feet, instead of biting into the wood safely.