VBS MoveFile Issue - file-io

Question

I did write the 1st if statement and it worked well in start while my file sizes were just in kbs.
as my file sizes got to mbs it started giving me problems like it was not renaming the file to .CSV again from .CSV.tmp hence I decided to add File Size before and after(2nd IF statement) , But it started to show me 2 debug msgBox "Entry in B Request with File : " and then "fname". after that it fails and don't give and error but don't do the job too.
do you find any issues with it?
Option Explicit
Dim FSO, FLD, FIL, GetFileSize1, GetFileSize2, f, fname
Dim strFolder,strFileName
strFolder = "\Sieb\Request"
Set FSO = CreateObject("Scripting.FileSystemObject")
set FLD = FSO.GetFolder(strFolder)
For Each Fil In FLD.Files
strFileName=Lcase(Fil.Name)
If instr (1,strFileName,"_a_request_")>0 then
if (FSO.FileExists(".\Sieb_Process_Files\a\Request\"+Fil.Name)) then
else
FSO.MoveFile ".\Request\"+Fil.Name,".\Sieb_Process_Files\a\Request\" + Fil.Name +".tmp"
FSO.MoveFile ".\Sieb_Process_Files\a\Request\" + Fil.Name +".tmp", ".\Sieb_Process_Files\a\Request\" + Fil.Name
end if
End if
If instr (1,strFileName,"_b_request_")>0 then
if (FSO.FileExists(".\Sieb_Process_Files\b\Request\"+Fil.Name)) then
else
MsgBox "Entry in B Request with File : " + Fil.Name
set fname = ".\Sieb_Process_Files\b\Request\" + Fil.Name
MsgBox fname
set f = FSO.GetFile(fname)
GetFileSize1 = f.size
MsgBox "Orignal File Size" + GetFileSize1
FSO.MoveFile ".\Request\"+Fil.Name,".\Sieb_Process_Files\b\Request\" + Fil.Name +".tmp"
set f = FSO.GetFile(".\Sieb_Process_Files\b\Request\" + Fil.Name +".tmp")
GetFileSize2 = f.size
MsgBox "Copied File Size" + GetFileSize2
MsgBox "File Moved with tmp name"
Do Until GetFileSize1=GetFileSize2
FSO.MoveFile ".\Sieb_Process_Files\b\Request\" + Fil.Name +".tmp" ,".\Sieb_Process_Files\b\Request\" + Fil.Name
MsgBox "File renamed to orignal name exiting now"
Exit Do
Loop
end if
End if
Next
Set Fil = nothing
set FSO = nothing

Answer 1

It's clearly not possible that your first If conditional ever successfully moved a file that didn't exist in the destination (and file size doesn't have anything to do with it).
if (FSO.FileExists(".\Sieb_Process_Files\a\Request\"+Fil.Name)) then
else
FSO.MoveFile ".\Request\"+Fil.Name,".\Sieb_Process_Files\a\Request\" + Fil.Name +".tmp"
FSO.MoveFile ".\Sieb_Process_Files\a\Request\" + Fil.Name +".tmp", ".\Sieb_Process_Files\a\Request\" + Fil.Name
end if
Once you move the file to a different location/name the file your variable Fil refers to doesn't exist anymore, so the second MoveFile (or rather, the attempt to access the property Fil.Name) would raise a "File not Found" error. If you didn't get an error you have an On Error Resume Next in your code that you didn't show.
Also, why are you moving the file to a temp name and then back to the original name in the same directory? That would only make sense when moving the file to a hot folder on a different volume, which doesn't seem to be the case here.
Something like this should be all you need:
Option Explicit
Dim fso, sourceFolder, targetFolder, f, target
sourceFolder = "\Sieb\Request"
targetFolder = ".\Sieb_Process_Files\a\Request"
Set fso = CreateObject("Scripting.FileSystemObject")
For Each f In fso.GetFolder(sourceFolder).Files
target = fso.BuildPath(targetFolder, f.Name)
If InStr(1, f.Name, "_a_request_", vbTextCompare) > 0 Then
If Not fso.FileExists(target) Then
f.Move target
End If
End If
Next
vbTextCompare makes InStr() comparisons case-insensitive, so you don't need to lowercase the filename.