You should not set the property ReadOnly="true" on your TextBox.
If TextBox's ReadOnly property is "true", postback data won't be loaded e.g it essentially means TextBox being readonly from server-side standpoint (client-side changes will be ignored). If you want TB to be readonly in the "old manner" use
TextBox1.Attributes.Add("readonly","readonly")
as that won't affect server-side functionality.
For more information follow StackoverflowAnswer or TextBox Readonly problem.