Alloy Picker not visible on screen

I am using the picker below to let the user pick a time period. Using a Genymotion emulator, I can see the picker. However, when it is installed on my Galaxy s4 or Moto G, it is not visible. I did not define a stylesheet for the picker. Is there anything you see that could explain my problem?

<Picker id="MinutePicker" top="50" selectionIndicator="true">
                    <PickerColumn id="column1">
                        <PickerRow value="30" title="30 Minutes"/>
                        <PickerRow value="60" title="60 Minutes"/>
                        <PickerRow value="90" title="90 Minutes"/>
                        <PickerRow value="120" title="120 Minutes"/>
                    </PickerColumn>   
            </Picker>