The spies focused on the French diplomatic code. This was the code or encryption used between Paris and the French embassy in London and elsewhere. The spies collected mountains of encrypted messages, but they couldn't break the code. They couldn't read the messages. They couldn't get inside information on what the French were thinking.
Some people look at the world differently than others.
Like sound engineers, for example. They noticed there seemed to be a faint second signal being transmitted along with the encrypted signal. They wondered what the second signal was all about, so they built some equipment to pull off the second signal in a usable form. That is, the French cipher machine was sending out not only the encrypted message traffic it was supposed to send out, but was inadvertently also sending along the original unencrypted traffic. The problem experienced by the French cipher machine is a general problem with computers.