hub inserted in ethernet cabling ruins communication

Hello, I have a system with an arduino shield (let us call it device A) with W5200 chip that is connected to a 3G router, and the router is connected (by ethernet cable again) to the ethernet port of an industrial computer that we shall call device B.
I wanted to analyze the traffic using wireshark. For so, I used to insert a hub in one of the cables so that all the traffic comes to my pc as well, and wireshark displays it. I did this many times.
What is new is that when I insert my hub between B and the router, everything works well, and I can analyze the traffic.
But, if I insert the hub between the device A and the router, the connection fails. I see the device A connecting to the router in TCP, sending it a query string as expected, then the router initiates the reply by sending SYN, the device A replies ACK, then the router sends SYN again and device A sends FIN. No reply can reach device A following a query to any device (either router or device B).
I thought that my W5200 was defective in some way, so I replaced the shield board with an another one, again using a W5200. Same problem again.
How can the W5200 fail working when a hub is inserted in its cable, whereas the other device is not affected? And again, I did this several times, and also I did this between two W5200, and it had always worked.
Any idea?