1. When users open your Facebook page from mobiles, they do not see the 'Shop' link.
This is a limitation of Facebook: links to 3rd party applications (i.e., those developed not by Facebook) are not shown on the Pages from mobiles. It affects all applications, not just Ecwid.
So when you access Facebook page from a mobile (and Facebook considers iPhones, iPads and Android tablets as mobile devices), you will not see a link to your Ecwid shop. Furthermore, if you try to open Ecwid store on Facebook in a mobile browser, it will display page not found error.
At present it is technically not possible to display standard Facebook tab applications on mobiles. We have to live with this restriction until Facebook changes it. Therefore, unfortunately we don't have a mobile version of the Ecwid Facebook app. Yet, we can offer you ways to work this around, see below.
2. One more restriction relates to relations between Facebook and Apple: "Add to bag" button isn't shown on mobile catalogs if opened in mobile Facebook application for iOS. This is a forced measure that we had to take due to the following Facebook restriction.
According to the Facebook policy: "If you’re using iOS to run your app, use an iOS approved payment method".
Facebook contacted us with this regard and requested to disable all payment methods in Ecwid application, otherwise our application would be closed.
In order to keep the Ecwid app running on thousands of Facebook pages, as a way to work this around we had to remove 'Add to Bag' button from the mobile catalog when it's viewed from mobile Facebook app on iOS.
Regrettably, it doesn't appear possible to sell anything in mobile Facebook iOS app unless items are sold within online Apple store.
The solution is here: Store on Facebook does not show on mobile, what should I do?