Don't know if you've tried hooked blades in your utility knife or not, but they've worked very well for me. I started using them after getting some Herman Oak leather due to the difficulty of cutting it and it helped me out a lot. It's something inexpensive to try if you haven't already. The link below is the style I'm talking about.
http://www.homedepot.com/p/Unbranded-5-Heavy-Duty-Hook-Utility-Knife-Blades-RWK16V/204156401