Lots of the treasure chests aren't actual chests. They're just something valuable.
Make sure you've got a free slot in your quest log or you won't be able to get the quest to get the treasure. Also there seems to be a thing, either level requirement or you can't be on another harvesting mission that might interfere with the ability to get a treasure. Not sure which, but sometimes the treasure just can't be gotten. One treasure a day, too, so if you got the chest in another area in the last 24 hours, that one will probably not work, either.