This is the story of my life. 
Have to make sure the lighting is just right. I have to press the start button 3 or 4 times before it will read the marks. Then, when it does, the cuts are off. So, if your cuts are perfect, I’d say keep hitting the start button. Also, it’s noted Here to make sure the rollers are in a specific spot on your mat. I just adjusted mine, the other day, but haven’t tried to PTC with it since. I got busy doing other things, but you can try that. Apparently, they help to read the marks.
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