c# - Updating UI on a UWP (Raspberry Pi / Windows Iot) -

i'm building c# uwp program (using mvvmlight framework) run on raspberry pi running windows 10 iot.

i have method downloads data sql database via wcf connection, can see debugging wcf service working, , data downloads.

i'm puzzled why can call method 2 different ways, 1 updates ui, 1 doesn't. please can explain why?

public class tagregstep2viewmodel : viewmodelbase {     private observablecollection<employeename> _employees;     private string _onscreeninstructions;     public asyncrelaycommand startwizardcommand { get; private set; }      public tagregstep2viewmodel()     {         // initialise messenger                     messenger.default.register<initialisestepmessage>(this, (msg) => initialisestep(msg.step));         // initialise commands         startwizardcommand = new asyncrelaycommand(() => getdata(), () => true);         //set dummy default value         onscreeninstructions = "default value";     }      // can see data downloaded wcf service , observablecollection updates, isn't reflected in ui.     public void initialisestep(viewmodelbase vm)      {         if (vm.gettype() == this.gettype())         {             debug.writeline("initialisestep() called");             startwizardcommand.tryexecute();         }     }      // method retrieves data wcf service.      // when called relay command, updates observablecollection , ui.      // when called messengercommand updates observablecollection not ui. why???      public async task getdata()     {         debug.writeline("getting data...");          onscreeninstructions = "loading employees";         await task.delay(25);          var emps = await loademployees(); //not posted method, works         employees = new observablecollection<employeename>(emps);          onscreeninstructions = "please select employee.";         await task.delay(25);          debug.writeline("observablecollection count :" + employees.count);      }      public observablecollection<employeename> employees     {         { return _employees; }         set { set(() => employees, ref _employees, value); }     }      public string onscreeninstructions     {         { return _onscreeninstructions; }         set { set(() => onscreeninstructions, ref _onscreeninstructions, value); }     }  }