Hi @manh189,
Without having a screenshot showing what type of items your module includes, we'll be guessing a bit at the answer here. Some items, like assignments/quizzes, are automatically marked as complete when they are submitted. Other things, like reading a page, have to be manually marked as "done" (read) by each student. It sounds like some students are following the process correctly, but maybe others are not aware of what they need to do or forgetting to mark as done, which would then leave the blank circles shown in your screenshot. I'd recommend sharing the guide I linked here with your students so they all know the steps they need to do to mark items as done.
Hope this info helps a bit!
-Chris