From my experience it could be a couple of things.
Is your cutting strip in good condition? They need to be replaced periodically, especially after you accidentally cut through the material and backing.
What type of material is it? Some brands and types are more consistent in thickness, some not.
Are you using the proper blade for that material?
Have you looked at your blade tip under a loupe?
Is the plastic housing around the tip of the blade clean of adhesive/bits of vinyl?
Lastly, the 8600 is an older machine at this point, it may need some servicing/cleaning. Dust and debris can keep all the tiny moving parts in the plotter head assembly from moving the blade up and down properly.