Items can only be picked up from the last item placed

You are not providing enough information for anyone to help you.

I have multiple items that are children of one master item blueprint class. For some reason, my character can only pick items in the reverse order they were placed (the last one placed can be picked up first, etc). I have absolutely no idea what could be causing this, so any help would be appreciated.

You need to provide more detail about your project and how you are approaching this. You have not provided enough information for anyone to help you.