The "next" button is not meant to add or go to a new page in the questionnaire builder. It represents the "next" button as it is being displayed to a respondent to navigate to the next page or previous page when he is actually filling out the live questionnaire. You can toggle this setting on/off in the page settings to the right.
If you wish to access a different page within the questionnaire builder, simply click on the designated page in the slider to the left.
If you wish to add a new page, please use the "add new page" button in the right top corner as shown within this screenshot:
