I sent the image off to cut, inserted the mat and pressed the Go button on the Imagine screen (as per the usual instructions on the CCR screen.) The mat was pulled into the Imagine but didn't cut. So I cancelled the cut and tried again. The same thing happened. To me it seemed that the Imagine couldn't work out where to cut the image.
I removed the mat and relocated the tape so that it wasn't covering any of the four registration marks on the mat. This time when I sent the file off to the Imagine it cut without a hitch!
So, if you're using tape to hold your cardstock to your Imagine mat make sure the tape doesn't cover any of the four registration marks.
Here's where you shouldn't place the tape (the same applies to the bottom of the mat):
And here's where you can place it (you can probably also place it anywhere on the border, just not on the registration marks):
Hope that helps!