I'm pleased to see you reaching out to Community. It's important that you're able to track your mileage for work trips and business expenses and I can see how this can greatly affect your books when it's not responding. The QuickBooks Online Mileage feature is wonderful and is only available for specific users on the account. I'd be happy to help you with this!
My first suggestion is to uninstall the app and then reinstall it to ensure it's up to date. In addition, as mentioned, only administrators on the account will have the ability to use this feature. Once you've confirmed your user access, check your device's location settings and make sure the QuickBooks mobile app can run in the background. If it's not running in the background it only tracks trips when it's open on your screen.
Follow these steps to check your settings:
On iOS go to your Settings
Locate and select the QuickBooks app
Choose Location and then Always
Allow the Motion & Fitness, Background app Refresh and Cellular Data options
When finished Close your settings
If you see a message about background location use, Always Allow