Why responses from mastodon.social don't arrive to my Lemmy instance?

I suppose this is a problem with the instance, but I have no idea how to debug it.

This is the post https://feddit.bg/post/292412 and this is a comment from a mastodon user that doesn't make it through: https://mastodon.social/@dimkorchev/115912556149075848

All the prerequisites seem to be there, so I suppose the problem is with my instance. Any ideas what I should debug to understand the problem better?