Subscribe/Unsubscribe icon not working

The "subscribe/unsubscribe eye" icon in the threads is not working properly... when i subscribe to a thread it turns green for a few seconds and then if i load the page again it turns grey again... if i click on the arrow the "subscribe/unsubscribe" option is right but the icon does not work, it's always set on "not-subscribed mode" (=grey eye). it's already been something like two or three days