Instagram has a very advanced spam filter, that works by checking a lots of factors (like the IP address you are using, hashtags, mentions, post originality) to detect spam posts/captions and they just simply remove it.
If the caption is still not visible after posting for every post, even after making unique captions, without mentions/hashtags - you should let your account rest a bit, because that means it got flagged and every post will be like that until the flag expires (usually 24-48 hours).

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