Based on your application we have created a sample which contains the ItemsSource as ObservableCollection and we are adding the data dynamically not from constructor, but the provided data labels loading properly in X axis. Please find the sample from the following location.
If you still face the problem, please share the following information, so that it will be help to reproduce the issue from our side.
1. Series Type (Like Column, Line, Bar…)
2. Primary axis type (CategoryAxis, NumericalAxis or DateTimeAxis)
3. Syncfusion Xamarin Studio version.
4. Xamarin.Forms version
5. The device name which you used to test.