Stamps?

I am having trouble understand the whole "stamp" thing. What I mean by stamp is the little images such as a the sythe verified lock and the pallet(see above). I was wondering because I had a blue party hat stamp, yet logged in the other day to find it is now missing. Anyone have this same problem or know how to fix it?

 

They are called miniranks.
The blue partyhat, or the pallet you have now are participation miniranks.
They are only awarded with activity per a certain timeframe. If you stop posting the required amount in that section, it will be automatically taken from you.