Read and write to log file - vb.net

Question

I have a program with a few parallel processes writing to a log file, tha main program is also reading the log and pooling it to a text field.
The thing is, reached a certain point, I miss the log and always read 0 bytes, though the processes keep writing it. This is my reader sub, where after some time, the line Dim n As Integer = stream.Read(bytes, numBytesRead, numBytesToRead) always gives 0:
Public Function readFile(ByRef stream As FileStream) As String
Dim bytes() As Byte = New Byte((stream.Length) - 1) {}
Dim numBytesToRead As Integer = CType(stream.Length, Integer)
Dim numBytesRead As Integer = 0
While (numBytesToRead > 0)
' Read may return anything from 0 to numBytesToRead.
Dim n As Integer = stream.Read(bytes, numBytesRead, numBytesToRead)
' Break when the end of the file is reached.
If (n = 0) Then
Exit While
End If
numBytesRead = (numBytesRead + n)
numBytesToRead = (numBytesToRead - n)
End While
numBytesToRead = bytes.Length
Return Encoding.UTF8.GetString(bytes)
End Function
The stream parameter is a global variable (global to the main Winform) declared as:
Private FSReaderLog As FileStream
And initialized (only in form load) as shown next, opening stream and trunking file content:
Public Function InitLog(ByVal ruta_fichero As String) As FileStream
Dim FS As New FileStream(ruta_fichero, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.ReadWrite)
FS.SetLength(0)
FS.Flush()
Return FS
End Function
What can be wrong in the reader?
Thank you