There are a couple of factors that can affect how the QuickBooks Online mobile app tracks mileage.
For instance, it uses GPS technology to automatically detect and track your mileage as soon as you start driving.
It automatically records the start and end points along with the total distance traveled, particularly when the app is open. Make sure to enable GPS or Location tracking all the time for the app to record all trips.
You can read more details about this feature in these articles:
Aside from that, enabling the While Using the App or Only While Using options on your mobile phone can prevent the app from tracking your mileage consistently.
You can refer to these articles from Apple and Google's support sites on how to change the settings, depending on the type of device that you're using:
Let me also share this article for additional troubleshooting steps to make sure all business trips are captured while using the app: Fix mileage tracking issues in the QuickBooks Online mobile app.
I’d like to provide you with this article for further assistance when tracking trips and vehicle expenses in QBO: Reimburse employees' mileage and vehicle expenses in QuickBooks Online.
Feel free to reach out to the Community if you have more inquiries about using QuickBooks Online for bookkeeping and managing business trips.