You probably got the first side fought down into the trough, now that part of the tire is fighting getting the 2nd one that deep.
An extra pair of hands to get the started side of the bead pushed down into the trough wouldn't hurt. Also, the head on my HF tire changer is quite large. Using irons instead will give you just a little more room to work with.
Unless your shop is quite warm let the assembly sit in the house for a few hours. A couple degrees warmer on the rubber can help a lot.
Edited by MiCarl, December 29, 2016 - 04:02 PM.