Many times, I have found the difficulty to be the bead I am working with "seems" to be too small in diameter because of friction with the rim is cumulative from the oppisite side of the rim. The solution is to start at the opposite side of where the difficulty is, pinch the tire and set the bead as near the center between the rim edges as possible. Work it around, never letting the tire relax, until complete. Usually that gives enough slack that the remainder can then be rolled over the edge.

The problem in my experience is the depth between the rim edges. Some wheels are deeper than others. The shallow ones seem more difficult because the tire can't move as far or as freely around the wheel.

Oddly enough, for me, kevlar/folding bead tires seem more difficult. Almost as if a certain amount of "initial stretch" is factored into the manufacture so it is proper size after it has been fully inflated.

+1 on Continentals being the most difficult. Hardshells being the worst!