You can search my threads about this. It is what Deno mentioned, but my research has identified a couple other culprits that may attribute to the issue. Just think about this.
1. What type of leather are you using? Tandy, Hermann Oak, W/C?
2. What is the location where the leather was cut, front or back, top or bottom?
3. What type of resist product did you use and how did you apply it? sponge, sheep wool, etc?
4. How may coats? How did you apply it? up and down, circular motion, etc
5. Where did you apply it? Outside, inside? What was the weather like? Windy, snowing, raining, etc?
5. How long did you let it dry? Where did you leave it to dry, Outside/inside?
6. Finally, the most important part, I believe, is what did it look like when you INITIALLY applied it to leather? Did you happen to see spots or runs of darker leather or lighter coverage? Almost like it was not adhearing to the leather?
I know this is too much information to think about when finishing, and I know that other masters on here just say that they apply it and move on, but I think for me it is about knowledge. The more you learn about the process the better you will get. Most importantly, learning about YOUR process. I ended up do some test pieces and was able to answer these question above.
Good luck and Happy Holidays!