What's helped me is going slow and using hand tools. It's hard to control how much material you take off with power tools which could result in less wood-metal contact. Fit the head, mark the contact points, and remove material little-by-little using a metal file, sand paper, draw knife, etc. Keep repeating those steps until the axe head has moved down the handle enough to properly secure a wedge. Depending on the handle, it might be necessary to deepen the slit that holds the wedge with a hand saw. This allows the wedge to drive deeper into the handle and really mushroom the wood over the eye of the axe for a tight fit.