Perusall automatically groups students in the course into smaller "discussion groups" (usually about 20 students, unless your instructor elected to form groups of a different size). You will only see the comments of other students in your group. This is to ensure that:
Each document has a different set of groups formed, so you get to interact with different people over the course of the semester.
As a consequence of this, if you are the first one in your group to comment on a particular document, then you won't see any other comments in the document even though other students in the class may have also started commenting. This is normal; you should start to see other students' comments as others in your group start reading and commenting on the document as well.