Customer's network lock out our data loggers for >4 hrs if they are disconnected then reconnected.

We design, make and install TCP/IP PoE data loggers. We have a customers where everything is working fine. However, if I disconnect a data logger and reconnect it can take up to 4 hours for our system to see it again. Works fine at every other customer and here at base, back in the room in 5 seconds. It's not a PoE issue as it's lit up like a Christmas tree, but it's not pingable and we cannot connect to it. Luckily our system recovers the data, but I need to get to the bottom of it. I'm due to visit them soon and was looking for pointers. Any help/pointers would be appreciated.