
I suggest you go to the same directory of the these Excel workbooks and check if there are any hidden file created with "~$" symbol with the same file name after users open Excel workbooks.
Please make sure users have enough permission for smb share folder.
Besides, here is a similar thread "Users NOT prompted file is already open by another user?", if users are accessing these files over the network on an SMB share, pleasse check the Group Policy settings for network drives mapping and make sure it is set to Update instead of Replace.
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