The form is not displaying on mobile devices?

In the older blueprint based themes, BigCommerce used separate theme files for mobile. Very few stores are still using these, but if your store still is, the app can still be added to these mobile themes with some manual intervention. 

Due to BigCommerce restrictions, external sources like apps, can't edit the mobile theme files unless they have been previously edited in the store admin.

There are two options to edit the footer.html theme file:

Option 1: If your store is using a responsive theme, the separate mobile theme can be turned off. View the BigCommerce help article on blueprint mobile themes.

Option 2: If you are continuing to use the separate mobile theme, The followings steps are needed to add the app to your mobile theme: 

• Download the mobile theme
• Add the following line to the bottom of Panels/Footer.html

       %%Panel.InStockNotifyPanel%%

• Re-upload the mobile theme

BigCommerce help article on blueprint mobile themes