I think it'll handle up to 24" rims or close to it if i'm not mistaken? A 285/70/18 or similar truck tire is basically a 32"
That usually only happens to me as the tire is 75% on, and with stiff tires like Dunlops. Sometimes I have to grab the tire and help turn it a bit to get that last part on.
If the rim is spinning on the jaws you need more air compression.... New tires can rip if you dont get them inside enough on the rim.. Be carefull with the duck bill and if all is set well hard tires or not you should be able to mount with no effort...