The most probable answer to this is that you are connecting to the PC Line-In with a MONO jack plug. Look closely at the end of the plug. If it has 1 black ring it's mono, if it has 2 black rings then it's stereo. If it has 2 black rings make sure that the plug is pushed firmly into the socket.

If you're connecting to a microphone socket make sure that it accepts a stereo input otherwise it won't make any difference if you use a mono or a stereo jack plug. It will either record both left and right channels in mono or just a single left or right channel.

Mono and Stereo plugs: