Two way binding is not working

Hello All,
I am trying to bind the text of a button with the Property,It's binding fine for the first time but next time onwards it's not updating even it's been updated.
I am using xamarin.forms shared project.
Xaml Code:
Text="{Binding SelectText, Mode=TwoWay,}"
ViewModel Class property:

public string SelectText
        {
            get
            {
                if (string.IsNullOrEmpty (this._selectText))
                    return "SellectAll";
                else {
                    return this._selectText;
                }
            }
            set
            {
                    this._selectText = value;
                    this.OnPropertyChanged("SelectText");
            }
        }

As xamarin is not able to parse updateSourceTrigger so i am unable to use that,Please help me if there is any alternative to that.
Thanks in advance.