Custom Button Is Not Visible to Users

Confirm the button is associated to a flow
Confirm the visual flow referenced in the button has been activated

As there is no specific security for a Custom button, there can be separate security settings for visual force or other objects that can trigger a button.

-Under the details page in setup, see if users have the 'Flow User' Enabled
-Alternative: Check to ensure the user has the 'Run Flow' permission granted

Once the permission is granted, and all previous steps have been taken to create a custom button and add it to the page layout, the button will be visible.