I have noticed that there are times when I have setup scanning to smb or folder on a pc that the scanning works and then it suddenly stops. In many situations, the port becomes blocked by some antivirus or antimalware or virus or some other program such as a vpn or a remote access program. This could occur quickly or when the computer reaches a point when the program or service starts up which may take quite a bit longer.
I have found the following utility has helped with detecting this port grabbing situation.
CurrPorts – Monitoring Opened TCP/IP ports / connections on your network.