It may be the casings. I know I have learned to always let casing soak in clean water over night after rinsing before using. It allows the casing to rehydrate, relax and expand easier. I have found that loading the tubes under water is easier also, but the water is a poor lubricate and the casings can drag on the tube while loading. I also make sure and lub the tub, o-rings and the meat chamber real well before loading the meat , casings, or attempting to load the casing.
Then again, I understand sometimes people complain about bad casing and I have no doubt they maybe. did you try more than one? I can't imagine a casing from me with this belly to be the same as one from Jennifer Aniston with her belly.
I use Butchers and Packers casings and have been very pleased. There are others like Syracuse also.
I know I didn't reference much but I hope something helps you. Maybe one of the sausage Pro's will be on soon.
Bon Chance.