I have tried (for the first time) posting my impression about a series and I have written it exactly like this:

I never liked shoujo mangas until I read this one. Now I know there are decent shoujo mangas out there. Unfortunately they are quite rare.

I latter checked series description and my comment was gone. Why? Is it a bug or I broke some rules?

I was too short is my guess and the rules on the length may have changed at some point which explains the short ones from back in the day.
Write one of twice (or more) the length and try again. Have fun.

I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb

I see. I have tried 3 time longer now and hopefully it would pass moderation.

Yup, you're fine now

A just ruler amongst tyrants