My assumption is that by "detect" you mean, use the external microphone as a line input rather than the internal microphone. If the camera is not using the external microphone, it is probably a connection issue between the camera and the microphone. Check the output of the microphone on some other device, like your computer or a field recorder, to be sure it is actually providing an output. It is also possible the connecting cable may be bad, or perhaps the contacts in the camera are not working. You will need to do some troubleshooting to determine which.
