Does the rim have a central trough? If so, is the entire bead in that central trough before you try to mount the tire? Are these tubeless tires?
Also, it takes two to tango. Tight tires are often a tire/rim combination problem, not a tire rpoblem. For example, for me mounting Hutchinson tubeless road tires on Shimano WH-6800 was an unbelievable exercise in frustrations, ripped up hands and broken levers. I later mounted a similar set of tires to WTB rims without issue. Some rims (like Shimano) are notoriously oversized.