Hi there. This is because creators can hold comments for review before they get posted. Learn more here: https://goo.gl/c7ChQU . Hope this clarifies.
Hmm, to help the right team better investigate, please login to the account experiencing the issue & send a feedback report directly from YouTube: https://goo.gl/WdI0BK . You may not receive an individual response back, but the details will help us identify & fix any issues.