How to view line comments in GitHub?

This is top result for "Can't view github comments" in google.

If you're here because you've left comments, and can see your own comments, but others cannnot, then check for a yellow pending label on your comments. Others cannot see your comment until you've completed the review process and selected whether you want to request changes. Once you click through a full review, then others will see your comments.

Note: I'm answering a slightly different question than the O.P., but I have a feeling that lots of other devs with this issue are hitting this stackoverflow question. I'm just hoping to help them. I myself had this issue and couldn't find a more relevant answer than this thread. :)

The line-comments depend on the commit that were made on.

If the person that made the pull request rebased that commit then you're not looking at what you were previously looking. It's a different commit, thus there are no comments.

Look on the 'Your Actions' tab and find a line in the actions' history were you commented on that commit, click that commit and you'll see the comments are still there.
If the commit was rebased, you wont find that commit were it used to be (some branch) and not on any repo probably (if it's not on another branch). It is just cached by git and github, until the garbage collector kicks in.