Cannot get port to close - vb.net

Question

I am trying to fix an app that uses TCP to receive files. Unfortunately, I do not have the source code for the sending application. The problem that I am having is that after the first file is received, the second one is sent by the sending application, but it is not being picked up by the receiving application.
I believe the issue is that the socket is not being closed after receiving the file. I have a method that should close it, but _socket.Connected = false, so nothing is done. However, if I check the port, it is still bound, even after the socket is closed.
Private Sub CloseSocket(ByVal disconnect As Boolean)
If Not (_socket Is Nothing) Then
If (_socket.Connected = True) Then
_socket.Shutdown(SocketShutdown.Both)
If(disconnect) Then
_socket.Disconnect(True)
End If
End If
_socket.Close()
_socket = Nothing
End If
End Sub
I realize that I have not included much code, but the Listen method is quite large and convoluted. I can get additional code if it will provide insight. Any assistance would be greatly appreciated.
EDITED
The code that is used to check the port status is below:
Private Shared Function TcpIpGetPortStatus(ByVal port As Int32) As String
Dim properties As NetworkInformation.IPGlobalProperties
Dim listeners As Net.IPEndPoint()
Dim local As Net.IPEndPoint
Dim connections As NetworkInformation.TcpConnectionInformation()
Dim t As NetworkInformation.TcpConnectionInformation
Dim portStatus As String
portStatus = "Disconnected"
properties = NetworkInformation.IPGlobalProperties.GetIPGlobalProperties()
listeners = properties.GetActiveTcpListeners()
Try
' Cycle through all listening TCP connections and find the one that is associated with port.
For Each local In listeners
If (local.Port = port) Then
portStatus = "Connected"
Exit For
End If
Next local
' Port wasn't in the listening state so check if it is established.
If (portStatus = "Disconnected") Then
properties = NetworkInformation.IPGlobalProperties.GetIPGlobalProperties()
connections = properties.GetActiveTcpConnections()
' Cycle through all active TCP connections and find the one that is associated with port.
For Each t In connections
If (t.LocalEndPoint.Port = port) Then
Select Case t.State
Case NetworkInformation.TcpState.Established
portStatus = "Connected"
Exit For
End Select
End If
Next t
End If
Catch ex As Exception
' Handle Exception...
End Try
Return portStatus
End Function
This function will return "Connected" when called. I've traced it through, and found that the port I was using was still bound. Again, thank you for all help.
ADDITION
I just ran this between two separate systems, using WireShark to capture the data. There is a Sender and Receiver that I do not have sourcecode for, and am trying to integrate with, and the Sender and Receiver that I am updating the code to get it to communicate properly with the existing ones. The stream sent is identical in both cases (except for datetime in the ACK). If I send a second message from the existing sender to the Receiver being upgraded, the message is sent to the port, but the Receiver in development is never notified, which hangs both programs.

Answer 1

After pouring over the code that was provided to me, I finally realized that it was closing and disconnecting the wrong socket. Rather than calling Shutdown and Disconnect on the socket that was used to receive the message, it was called on the listening socket.
Once i realized this, I was able to close the correct socket, and the application began to work. Now I just need to go through and correct all of the issues that this caused with the underlying code, and things should be good.
Thank you to all of the people who responded. You got me thinking in the right direction.