In Bastion, some levels have items lying on the ground. Some of them merely provide NPC dialog, others are weapon upgrade materials, all of them give a small XP bonus.
The levels I've already cleared are marked with a red star on the map, and I can't revisit them.
If I clear a level without picking up all the items, are they lost forever, or would I be able to replay the level in this case? Can I miss out on weapon upgrades by not being thorough enough?