Page Builder tab is missing

Hi,
I am having a problem with the page builder editor. When editing a page, the page builder editor tab was located at the top right alongside the ‘visual’ and ‘text’ tabs but it is missing now. I have reinstalled the plugin but does not fixed the problem. They’ve actually moved to the bottom of the page, without any content in there. Currently, I have the latest version of WordPress (4.7.4) and Page Builder (2.5.2).