Why do i sometimes do not get exp whenever i post a threat or reply to somebody's thread?

Yesterday i recently discovered that replied to a thread and didn't get exp, what should i do or what did i do wrong?

Think there's a cap on the amount of XP that you can earn on a day.
It would be great if someone can confirm this as this is just my speculation.

Oh ok thanks for the explaination

There is a limited number of comments per day that gets you xp indeed. To "avoid spam" (or more precisely make it less severe).
From what I've seen, this is 5 comments a day (knowing you have to actually wait 24 hours, it doesn't reset the count at a specified time of the day (sadly)). Also, it seems it was 10 comments a day, before, but I can't tell since I wasn't there yet (or at least wasn't posting comments yet. It doesn't matter much anyway since that's not true anymore).

EDIT : Forgot, but just remembered that you can actually get this information by talking with Misty :
"I have a problem with the platform" → "Articles, forum" → "I'm active on the forum and I don't get any experience, why?"
► "There are only 5 posts for every 24 hours in order to avoid spam."