When my mat gets a crack in it it won't cut fabric where the crack is but if you are using a new mat, it's probably the die. It is more then likely one teeny tiny edge of the die blade isn't touching the fabric. Call and ask for a replacement die.
The mats are sold separate from the dies. One mat will fit many dies so it's not practical to have a mat with every new die. The mats are inexpensive anyway. I don't baby my Go but I usually only cut 4 layers of fabric and many times I put a thin piece of paper over the die underneath the fabric to keep the fabric from stretching when I'm cutting shapes from scrap pieces and may not be on grain.