Why isn’t my Facebook store showing on mobile?
The way that Facebook structures their mobile app means that no Facebook apps are visible when someone looks at a Facebook Page via the Facebook mobile site, or through the Facebook mobile app. The Selz store on Facebook is an app installed on your business page, which is why it's not visible from your mobile device.
Via the Facebook mobile app, you see the business page, but not any of your installed apps, including your Selz store.
Is there any way around this?
There is a way around this. Check out the article Selling on Facebook learn how to add a 'Shop Now' button to your page.