If the Publishing feature is activated on the SharePoint site, Changes made in master page are not visible to every users even if the changed master file is checked in. It must be approved by Site Collection Administrator.
To make all changes visible to every user,
1)We need to select "Publish a major version" option while 'checking in' the master page file (in SharePoint Designer).
2)Log in with the Site Collection Administrator on SharePoint Site navigate the 'Site Settings' page and click on 'Master Pages' under 'Web Designer Galleries'.
3)Find the master page that we have changed. In the ECB (Edit control Block) menu, click on 'Approve/Reject' option.
4)After approving the changes made in the master page will be visible to all users.