Interesting! I have always taught not to re-cut a previous line and this might be your problem. If you are cutting the leather while dry, you will have problems, I would think. When you properly wet your leather to start your second knife cuts, you will cause the fibers to contract and your previously cut lines may close up. I would think if your knife is dragging even after properly moistening your leather, it is probably your knife tip. I would still suggest that you sharpen your knife blade.
Here is another suggestion I would make is for you to use a stylus to draw your pattern out on the leather. When you are ready to work on the project, you wet the leather and go about your cutting after you properly sharpen and strop your knife before use. Just a suggestion.