okay i had found more information about this issue and the problem is it don't sync when you move server to server (like the way bungeecord works) so in the code it's bascially not syncing the right way and cause the backpack to not show on your back here is the code that sync when player login
https://github.com/runescapejon/AdventureBackpack2/blob/3e11048165cd3825a32ab93893b09b1671dc1271/src/main/java/com/darkona/adventurebackpack/handlers/PlayerEventHandler.java#L87-L94
again sorry but can one of you guys can help me about this problem