WPF UserControl - cannot select TextBox

I have a really simple WPF UserControl:

<UserControl x:Class="dr.SitecoreCompare.WPF.ConnectionEntry"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
x:Name="connEntry" 
BorderBrush="Navy" BorderThickness="1" Margin="5,0,0,5" >
<StackPanel Margin="0,10,0,0" >
    <Label FontWeight="ExtraBold" Content="{Binding ElementName=connEntry, Path=Title}"></Label>
    <Label Margin="0,5,0,0">Server:</Label>
    <TextBox x:Name="txtServer" TabIndex="1" Text="{Binding Path=ServerName}" ></TextBox>
    <Label>Database:</Label>
    <TextBox x:Name="txtDatabase" TabIndex="2" Text="{Binding Path=DatabaseName}"></TextBox>
</StackPanel>

      

        
        
        
      

    

This is used twice in the same window. Now I can select the first TextBox for both instances of my UserControl, but the second textbox (txtDatabase) cannot be selected by either tab or click. Why is this? Am I missing something in regards to creating custom WPF controls?

EDIT: DatabaseName is not readonly, it's a simple property. The XAML for the window where the usercontrol is installed looks like this:

<Window x:Class="dr.SitecoreCompare.WPF.ProjectDialog"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:c="clr-namespace:dr.SitecoreCompare.WPF"       
    Title="Choose project" Height="280" Width="500" 
    WindowStartupLocation="CenterOwner" WindowStyle="SingleBorderWindow" HorizontalAlignment="Center" ShowInTaskbar="False" ShowActivated="True" ResizeMode="NoResize" VerticalContentAlignment="Top" VerticalAlignment="Center">
    <StackPanel>
        <Label>Choose databases</Label>
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition />
                <ColumnDefinition />
            </Grid.ColumnDefinitions>
            <c:ConnectionEntry Grid.Column="0" x:Name="connMaster" Title="Master:" Padding="5" />
            <c:ConnectionEntry Grid.Column="1" x:Name="connSlave"  Title="Slave:"  Padding="5" />
        </Grid>
        <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Margin="0,0" >
            <Button x:Name="btnCancel" Click="btnCancel_Click">Cancel</Button>
            <Button x:Name="btnOK" Click="btnOK_Click">OK</Button>
        </StackPanel>
    </StackPanel>

</Window>