Some Themes are not supporting apps on featured products.
For example this is the case on the OS 2.0 DAWN Theme.

For all free and OS 1.0 Themes, you have to add a code snippet.
This should be placed inside the product form in sections/featured-product.liquid. 
If that template doesn’t exist, it may have another name.

Note: This is an instruction for OS 1.0 Themes.
For OS 2.0 Themes you have to add an App Block if possible.

Steps to follow:

Go to your Shopify Admin

1.    Navigate to Online Store

2.    Select Themes

3.    Navigate to Actions

4.    Click on Edit code

Search the sections/featured-product.liquid. 
Open the .liquid file and search the add to cart button.
Often the button is called like this: <button type=“submit“ name=“add“

Copy and paste the following code above the add to cart section:

<div class="textfieldapp" data-textfieldapp-product-json="{{ product | json | escape }}" data-textfieldapp-collections-json="{{ product.collections | json | escape }}"></div>