Encounter "Game Full" while muling

ok this is 1 thing i dont understand..i was planning to transfer a Ist rune so i joined a game that is called "act2 help plz" i went on a wp at act 5 and dropped the rune on the corner where noone will get its whereabout then i exited the game and took my other char then boom! game full i had a friend in the game saying its not full cuz there's only 3 ppl there and max is 8 but i still keep gettin gamu full msg costed me ist rune cuz when i was able to return to the game after 20 minutes of trying it was gone...im wondering if runes disappears when its been on ground for too long..
???