For anyone still curious about an answer, I was missing one bone from YEARS ago when I stopped playing. I had gotten the rest, beaten the level, and moved onto the next. Was afraid I would have to try with a new save. After coming back, I reloaded the save, went to play the game, and it gave me a chapter select. Tried the balloon level again, everything but the air tanks and one bone that was right near a short wall at the bottom.
So, if you complete the level, it saves what you've collected, removes them from future plays, and leaves only the uncollected ones. You MUST complete the level, as otherwise it doesn't register you having collected the bones.
Funny thing, said bone was one of the reasons I moved on to other games and didn't come back.
AM WILLING TO HELP WITH ACHIEVES.
Send messages to inquire. Some games I no longer have. Ones I have I can help with.
Please include information in the message, and not just a friend request.
East Coast, USA (EST/EDT timezone)
Available gaming/boosting times:
Weekdays: 7:00PM - 12:00PM (EST/EDT)
Weekends: Generally all day (EST/EDT)