When QuickBooks Multi User Mode Not Working, it typically indicates network or configuration issues preventing simultaneous access. Common causes include improper hosting settings, incorrect folder permissions, or firewall restrictions. Resolving this involves verifying that the hosting mode is enabled on the server computer, ensuring all users have proper access permissions, and configuring firewall settings to allow QuickBooks communication. Addressing these issues promptly helps restore seamless collaboration and data sharing across multiple users. For persistent issues, consult QuickBooks support or a certified professional for assistance at 1.855.409.2300.