When a customer buy the full course and pasted the first quizz, on the results page the link to return on the course goes to the first small course and te customer need to buy this part...
Ok, I am assuming the following:
You have one big course and instead of creating one course you created Course Part 1, Course Part 2 and Course part 3 as separate courses.
Please correct me if my above assumption is incorrect.
Answer:
Each of these courses are treated as a standalone course in WPLMS. Every course has unique ID value.
When you create quiz, you can select which course the quiz belongs to. It may be a case wher you selected an incorrect course in the quiz settings.
Solution :
a. Edit the relevant quiz in WP Admin
b. Locate the connected course in Quiz settings.
c. Correct the course and update the quiz.
Let me know if this does not help.