UART only works after reconnecting


I try to use UART on BBB but I have a problem with UART. We use two BBB and two computers.
I have two programs in Python using Pybbio library. One send data and the other receive data. Each program is on a different computer.
When I run both program at the same time, nothing happens. I have to move wires* (sometimes connect and disconnect them) and then, the transmission starts.
I connected ground between BBB.
I don’t understand why I have to disconnect wire.

Any idea?

*TXD and RXD