Best Practices

I have come across multiple articles about how there will seem to be a phantom lock on a document (network shared) by another user. I'm having the opposite problem. I have a shared excel document that doesn't show locked by any users when multiple users have it open. This is creating problems when another user overwrites another user's work. None of the document protection settings are turned up, and this wasn't a problem as of a couple weeks ago. The behavior I expect to see when a user opens a document that's already opened by another user is that they will get the locked for editing message. This does not happen. I haven't been able to find a specific article that addresses this problem, so I'm looking to the all-wise Spiceworks community for advice. Thank you in advance for your suggestions!