Loop to print text files is skipping some files(randomly, it seems) - vb.net

Question

I have a VB.NET program which lists some text files in a directory and loops through them. For each file, the program calls notepad.exe with the /p parameter and the filename to print the file, then copies the file to a history directory, sleeps for 5 seconds(to allow notepad to open and print), and finally deletes the original file.
What's happening is, instead of printing every single text file, it is only printing "random" files from the directory. Every single text file gets copied to the history directory and deleted from the original however, so I know that it is definitely listing all of the files and attempting to process each one. I've tried adding a call to Thread.Sleep for 5000 milliseconds, then changed it to 10000 milliseconds to be sure that the original file wasn't getting deleted before notepad grabbed it to print.
I'm more curious about what is actually happening than anything (a fix would be nice too!). I manually moved some of the files that did not print to the original directory, removing them from the history directory, and reran the program, where they DID print as they should, so I know it shouldn't be the files themselves, but something to do with the code.
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim f() As String = ListFiles("l:\", "997")
Dim i As Integer
Try
For i = 0 To f.Length - 1
If Not f(i) = "" Then
System.Diagnostics.Process.Start("Notepad.exe", " /p l:\" & f(i))
My.Computer.FileSystem.CopyFile("l:\" & f(i), "k:\" & f(i))
'Thread.Sleep(5000)
Thread.Sleep(10000)
My.Computer.FileSystem.DeleteFile("l:\" & f(i))
End If
Next
'Thread.Sleep(5000)
Thread.Sleep(10000)
Catch ex As Exception
End Try
End Sub
Public Function ListFiles(ByVal strFilePath As String, ByVal strFileFilter As String) As String()
'finds all files in the strFilePath variable and matches them to the strFileFilter variable
'adds to string array strFiles if filename matches filter
Dim i As Integer = 0
Dim strFileName As String
Dim strFiles(0) As String
Dim strExclude As String = ""
Dim pos As Integer = 0
Dim posinc As Integer = 0
strFileName = Dir(strFilePath)
Do While Len(strFileName) > 0
'check to see if filename matches filter
If InStr(strFileName, strFileFilter) Then
If InStr(strFileName, "997") Then
FileOpen(1, strFilePath & strFileName, OpenMode.Input)
Do Until EOF(1)
strExclude = InputString(1, LOF(1))
Loop
pos = InStr(UCase(strExclude), "MANIFEST")
posinc = posinc + pos
pos = InStr(UCase(strExclude), "INVOICE")
posinc = posinc + pos
FileClose(1)
Else : posinc = 1
End If
If posinc > 0 Then
'add file to array
ReDim Preserve strFiles(i)
strFiles(i) = strFileName
i += 1
Else
My.Computer.FileSystem.MoveFile("l:\" & strFileName, "k:\" & strFileName)
End If
'MsgBox(strFileName & " " & IO.File.GetLastWriteTime(strFileName).ToString)
pos = 0
posinc = 0
End If
'get the next file
strFileName = Dir()
Loop
Return strFiles
End Function
Brief overview of the code above. An automated program fills the "L:\" directory with text files, and this program needs to print out certain files with "997" in the filename (namely files with "997" in the filename and containing the text "INVOICE" or "MANIFEST"). The ListFiles function does exactly this, then back in the Form1_Load() sub it is supposed to print each file, copy it, and delete the original.
Something to note, this code is developed in Visual Studio 2013 on Windows 7. The machine that actually runs this program is still on Windows XP.

Answer 1

I can see a few issues. the first and most obvious is the error handling:
You have a Try.. Catch with no error handling. You may be running in to an error without knowing it!! Add some output here, so you know if that is the case.
The second issue is to do with the way you are handling Process classes.
Instead of just calling System.Diagnostics.Process.Start in a loop and sleeping you should use the inbuilt method of handling execution. You are also not disposing of anything, which makes me die a little inside.
Try something like
Using p As New System.Diagnostics.Process
p.Start("Notepad.exe", " /p l:\" & f(i))
p.WaitForExit()
End Using
With both of these changes in place you should not have any issues. If you do there should at least be errors for you to look at and provide here, if necessary.