You're unable to match the expenses in your QuickBooks Online mobile application if the device is full of cache and cookies. With this, I'd recommend doing primary troubleshooting steps to isolate the issue in your app.
To start, I'd suggest logging into your account via browser and match them. That way, we can verify if there's something wrong with your app. See Compatible browsers for tablet/mobile devices for more information on supported browsers for mobile devices.
If you're able to do so using a browser, let's get your mobile app up and running by resetting the app. This is the same as clearing the cache from your web browser.
But before doing that ensure that you're using iOS 13.0 or later. Once confirmed and the issue continues, here's how to rest the app.
Go to the Help menu.
Choose the Refresh Data menu.
Hit Yes.
After that, please re-match the expense to verify if it's already functioning adequately. If the issue persists, let's uninstall and reinstall the app for a clean slate. You need not worry since it won't delete any of your app's transactions. You can touch and hold the app, then click the Delete app button.