We may see this issue if the template does not have a set layout for mobile devices. To resolve this issue, we can set the layout for mobile devices: 
  1. Log into NetCommunity.
  2. Navigate to the event page in question > Select Edit this page.
  3. Click the Template designer button to access the template in use for this page.
  4. Click Template properties.
  5. Under Mobile devices, set the specific layout to use for mobile.
  • Note if we have this set as 'Use same as desktop', select the specific layout in use for the desktop as the layout to use for mobile.
  1. Click Save
  2. Navigate to Administration > Sites & Settings.
  3. Select the Schedules tab
  4. Scroll to Secure Payment Template Update and click Update Templates 
  5. Click Save
If after running the Secure Payment Updater, the payment screen still does not still as it should, this indicates a design-related issue with the template. How do I fix design issues or make changes to the design on my site?

Additional Resources:
How to Enable or run the Secure Payment Template Updates 
Payment page for event registration or donation form is not showing the correct template