The 12mm 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.