I have made snack sticks for many years, venison as well as beef. The first few times I made them, I had a similar problem. During the smoking process the collagen casings would pull away from the meat as it dried. I would have to strip the sticks to have a presentable product so others would not be afraid to try what looked like a mistake.
I found that after stuffing them, let them dry for two to six hours uncovered in the refrigerator. The moisture passes through the collagen and the casing begins to adhere to the meat. Now when you smoke, the casing shrinks with the meat and will resemble a "Slim Jim" in appearance. This makes a much more presentable product. The drying time becomes dependent upon the amount of water added to the sausage mixture.
The Sausage Maker in Buffalo, NY (http://www.sausagemaker.com
) also sells a casing pricker to help the drying of sausages.