Crash data isn’t uploaded to App Center until after the user restarts the crashed app. During a crash, the app is busy crashing, so there’s no reliable way to deliver the data until the app’s running in a stable state again after restart. This means that if your user abandons, or delays restarting the app, you’ll either never see the data or see it much later than expected.

For iOS applications, the most common reason for crash data not appearing in App Center is because the app’s symbols aren't uploaded to App Center or because Bitcode is enabled. Details for uploading symbols are available at https://docs.microsoft.com/en-us/appcenter/diagnostics/iOS-symbolication.