Before starting, please make sure that you have the latest version of Thymio Suite (re-install Thymio Suite if you are not sure) and that your USB cable is not defective by trying to use another one.
If you are on Windows 10, The problem is likely due to a faulty installation of Bonjour service.
1. On Windows, uninstall Bonjour. Open the Windows 10 Settings, Apps & Features. Locate "Bonjour" in the list and uninstall it.
2. Download the Bonjour installer from our github repositery (Bonjour.msi for 32 bits versions of Windows, Bonjour64.msi for 64 bits versions).
3. Install Bonjour.
4. Restart Thymio Suite.
5. In addition, you can check that both "Bonjour Service" and "thymio-device-manager" are allowed through the firewall. Open the Windows security center, Firewall & Network Protection, and click on "Allow an app through Firewall". Check in the list that the boxes are ticked for those two applications.