in order to help us fix your problem, you'll have to understand how Bandicam works a little.
You see, Bandicam doesn't actually record the Windows System sound or a Game or a Mic,
It records everything that is sent to a set audio device (which can be both, an output or input device, like speakers or microphones) as they are selected in Bandicam and the control panel.
That means, that your microphone cannot be recorded from, if it is not selected in Bandicam, except it playing back through your main speakers.
Now with this knowledge, you might find out what causes your problem a little easier.
Hope I was able to help you,
Thank you for reading my post. I hope you understand my "English".
I'm the German translator for some of Bandisoft's products and websites
(mostly Bandicut and Honeycam).