Some comment sections are locked for no apparent reason.
For example, comments for "La esponja" (The sponge) in Spanish are locked, without a single comment posted in it!
I found several other comment sections locked in Spanish, with comments in them but with no apparent reason for the lock. :/
Hi there, did you have a specific comment you wanted to make for La esponja?
The Spanish course is one of Duolingo's most seasoned courses. Generally, those sentence discussions are only locked when we see people posting clutter such as casual comments that aren't helpful to people learning the language, repeating questions that others have already asked, spam, profanity, and things like that. Some discussions collect over 800 comments and after we clean them less than 50 were useful. Some discussions have collected nothing useful but take a lot of time to keep cleaning. So, usually, the reasons aren't apparent because we've cleaned them up. But, there usually is a reason that was apparent to a moderator or staff member.
Thanks for your concern!