I've never mounted up that tire, but with Bridgestones, Continentals, Pirellis and Shinkos it's been no problem with liberal application of Ru-Glyde during mounting. How much pressure are you using? Sometimes you have to use a bit more than is "comfortable" to get the bead to seat.
I do sometimes have issues with getting tires to seat on the Guzzi cast rims. The "secret weapon" there seems to be Windex liberally applied and lots of pressure.
If you're doing this in a cold shop, try warming the tire somehow. I lay mine on the workbench closest the woodstove.