Label.Text doesn't change in a loop - vb.net

Question

I'm trying to change a text that a label displays during each iteration of a Do While loop in Visual Basic. One label (which displays an integer) works fine, however the other stays blank until the loop finishes and displays the final result. What could be the problem?
Private Sub btnCalc_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
Handles btnOblicz.Click
Dim W As Single
Dim L As Integer
Dim J As Integer
Dim Y As Double
W = Val(txtW.Text)
L = InputBox("Input L")
J = 0
If W > 0 And W < 100 Then
Do
Y = (2 * Math.Sqrt(W)) / (L - J)
J = J + 1
lblJ.Text = Str(J)
lblY.Text = Str(Y)
MsgBox("Next...")
Loop Until Y < 0
Else
MsgBox("No calculations, because the number is less than zero or greater than a hundred.")
End If
End Sub

Answer 1

Application.DoEvents(), as suggested by others, can work. However, you should be aware that it has some negative side-effects, such as the potential to create StackOverflowExceptions.
The right way to solve this problem is to use a BackgroundWorker component.
As to why this happens... remember that all windows programs work by having, at their core, a loop which checks for messages from the user and operating system (things like mousemove events, clicks, keystrokes, etc). When you set the text property of a label, you are not telling the label to re-draw itself on the screen. Instead, you are posting an event to the operating system that your program's message loop must then receive and process. As new events come in, the message loop (or pump) sends those events to the proper method.
Your btnCalc_Click() function is one such method. If your function is running, it was called by the core windows messaging loop, which is now waiting for your method to complete and return control: it's blocked. The loop cannot continue receiving and dispatching methods until your function completes, and therefore nothing in your program's interface can be updated.

Answer 2

This worked fine for me in small copy I did using VS2010
It may be that a message pump is required. Try this :
System.Windows.Forms.Application.DoEvents()

Answer 3

Probably, your window function which processes OS event message is inaccesible while your loop executes. Try explicitly call dispatching messages. E.g. insert after assigning labels text inside your loop
Application.DoEvents