You can confirm that you’re in multi-user mode by checking the top left portion of the QuickBooks window:
In order for multiple users to access the same QuickBooks company file, all users must be in ‘multi-user’ mode. You can also select File/Switch to multi-user mode; however, the next time that you log into the company file, it will return to single-user mode. To ensure that the company file always opens in multi-user mode, select Open a company file / Next (on open a company file), then place a checkmark in the box at the bottom ‘open in multi-user’ mode.
It will continue to open in multi-user mode until File/Switch to Single-user mode is selected. At that point it will open in single-user mode.