The system reports the comment as sent and nothing appears under the post. The send did happen — the question is what became of the message afterwards.
Pre-moderation
Some discussion groups hold comments for an admin. The sender sees their own message, nobody else does until it is approved, and a share never gets approved.
Automated filters
Moderator bots delete messages by pattern: a link, a mention, an unfamiliar member, a first message in the group. The comment lives for seconds. The giveaway is that the ones with links are the ones that vanish.
Shadow restriction
An account under a spam restriction keeps sending, and nobody except the sender sees the result. The nastiest case, because everything looks fine from the inside. A status check settles it.
You are looking in the wrong place
Comments live in the linked discussion group, not in the channel. Looking in the channel, you will not find them.
How to tell them apart
Open the discussion from an account that is not involved. Visible means your client is at fault; invisible means work down the list — account status first, then the group's rules. Removing links from comments helps in every case: they are the first thing filters catch. How commenting works is on the automation page.