Chalk can fog up the sensor but since you have cleaned it, that shouldn't be the problem. I had one black fabric that gave me fits! I finally switched to my regular darning foot and it stitched fine. How full is your bobbin? Mine likes a fairly full bobbin and can get grumpy as it nears the end. Try a different spool of thread as well.
Have you broken a needle lately? Sometimes that can knock off the timing and in that case it needs a trip to the shop. Clean out your bobbin area and add a drop of oil to the hook even if it isn't time to do so. Make sure the thread from the thread spool is feeding smoothly as a tiny catch can cause problems.