Make sure you go to Settings and select any languages you want to see as well as Undetermined - I had a similar issue which was fixed by doing this.
If that’s not it, it may be a delay in comments federating if it’s a community you just recently subscribed to, or there could be federation issues. If the post is on an instance that’s defederated with yours, I don’t think it would show up at all, unless the defederation happened after it was made.
You can check who an instance has defederated with by checking the Instances link at the bottom of the mane page (or adding /instances after the URL). There’s also a tool to see what other instances have defederated with a particular one here: defed.xyz