The 12mm, 15mm and 17mm hole cutting dies require a lot more pressure compared to the smaller ones. The most common user error for that die is that not enough pressure is being applied. We recommend placing your press on a surface low enough so that you can fully straighten your elbow when you press down on the handle. This will allow you to lean your body weight into the press.
If your webbing or other material is very dense, you may not be able to apply sufficient pressure for that.
But if you test it on a few layers of fabric (say 4 layers) or vinyl, it should work with the right amount of pressure. Typically, you will hear and feel a "give" in the spring when enough pressure is applied. If you don't experience that, then there's just not enough pressure.
You can also try adding an additional layer or two of scrap vinyl underneath the material you actually want to cut. This will add some necessary compression so that the top layers are sufficiently cut. (The bottom scrap layers may not cut fully.)
