Some elements (text/image/video) automatically "carry over" to the next slide and that's why you can't edit those elements. 

You can check out the bottom of each slide and see which elements are editable. For example, if there are both 🇹icon and 📹icons, you can edit the text and video of the slide. However, if there is only a 🇹icon, you can edit the text only.

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