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.
  6. Save.
  7. Navigate to Administration > Sites & Settings.
  8. Select the schedules tab.
  9. Click Update Templates under the Secure payment template update heading.
  10. Save

Please refer to, how to run  the Secure Payment Template Updates for more information about this process.  After the updates have completed, the correct template should now display for the event payment page on mobile devices.

Payment page for event registration or donation form is not showing the correct template