My guess it that you are flaring the case too much and the mouth is scrapping the inside of the sizing die as it is pulled out. I used to run into that when I was trying to increase the flare to seat an oversized lead bullet. You most likely are flaring the case so much that the expander plug is pushing the mouth out against the inside of the die body.
Try starting with a new ly sized case and back your expander plug out about a turn. Then, slowly add flair until the bullet sit 1/32-1/16 into the case mouth. That's all you need and it shouldn't cause you to feel resistance on the down stroke.
Normal expander plugs work but they are not ideal. I would suggest looking at dies with a 2-step expander plug like that founf in Lyman M-type Expander dies, Redding, and Dillion. Instead of just belling the case mouth, they actula create a shelf for the bullet to sit in. It not only helps you seat the bullet straight but you don't have to worry about over-expanding the case mouth.
http://forum.accurateshooter.com/threads/lyman-m-expanding-die.3990627/