When should I contact my instructor if I experience problems?
As soon as possible!
Remember that if you don't tell your instructor that anything is wrong, and stop showing up to class, they may assume that you are simply skipping class.
If you need help, or are having some kind of trouble, your best bet is to keep your instructor informed and up-to-date.
If you are having technical or other problems during the semester, you can always call or email your instructor.