Make sure you are inside your Interactive Deck Editor, then follow below steps:
Step 1: Go to Home tab inside your deck editor.
Step 2: Now using slide navigation menu, move to the slide, after which you want to create a new slide
Step 3: Now you can create a new slide with two ways.
i) Click on “New Slide” button or
ii) Click on “+” icon in the webform on current page
Step 4: Check that the new slide is created at the desired position.