The most common cause is a blunt needle, I’d use a 16 for canvas.
If it’s not the needle have you checked your threading, both needle and bobbin? Any stray threads or fluff in the bobbin case?
Is your bobbin wound properly? Sometimes mine can be a bit loose and that can cause problems.
Was the machine stitching properly before you started top stitching?
I try not to top stitching too close to the edge, a millimetre or so further from edge can sometimes help.