Make sure both beads are all the way down into the center "trough" of the rim as you work the last piece of bead over the lip of the wheel. Obviously, seat both sides of the bead at the valve stem first. But no matter how many times you do it, and how well refined your technique, some rims just don't like some tires. I tried to mount a Vittoria tire on my old WTB rims, and it simply was not going to happen. The Hutchinsons (tubeless) on my wife's bike were absolute monsters to get on-- the meanest tires I've ever mounted. Meanwhile, my Maxxis ReFuse practically fall onto the rims.