maybe they got jumbled somehow - if you put one inadvertently within another that would mess it up
simply each one has the <ref> before it and the </ref> after it, so click edit and scroll down, some folks like to use a new line for each on the edit page so they stand out and that way you can see and find each one and make sure it is matched up with its partner - have to have the front and back for each source citation - and the C thing should do that but sometimes we accidentally click in the wrong place before we hit that C so it goes in in between another set of front and back for another source - pretty sure that is it - I have done it and it left me confused until I found it - spread them out by hitting return so each is on its own line to help find them if you like
hope that makes sense to you