Be sure you check the published departmental and course policies early in the course if you anticipate a problem. Most instructors will not be very understanding if you fail to read the information they have already provided for you, or if you also miss the scheduled make-up date. Also, you will probably have better luck if you ask to take a test early, based on a schedule conflict, rather than asking to take it later. If you still find reluctance when you have a compelling schedule problem ask if you may take the test for slightly fewer points. This may convince the professor of your compelling need.
Some courses are just hard.
It may be the nature of the course work, or simply a subject matter that isn't your favorite. If you need help on a course there are several places to turn. First ask for assistance from the instructor, if you find you need more help, ask for referral to the school's tutoring process. Many schools are beginning to establish special programs for students at a distance. Third, using a tried and true method for years, you may find assistance among other class members.