Sometimes Messagebox.show Cannot Appear On Screen

I am using visual basic .net 2005 win application.

My code,

Dim
coyCollect As
New Collections.Specialized.StringCollection
Dim

[code]....

i am sending list of company codes to the function. and in function i loop through to do something. if success i will return 1 if some error occures i will return 2)

If Res = 1
Then
MessageBox.Show(
"Success.",
"My App"

[code]....

My probelm, sometimes messagebox.show method don't show messagebox on screen. It will appear in task bar but we have to click it to show it on screen (looks like it is hanged but messagebox is already there in taskbar so click it to show message "success" then click ok to close the messagebox). then only we can click "ok" to close it. without clicking it my "in progress" window won't close. sometimes messagebox will appear with no problem.