When you click on the record button you are asked to provide a filename for the Voiceover.

If you leave a space in the filename or put a file extension on the file when creating it the recording will not appear in the listing when you have recorded.
For example
Bad names to use are
My Voice Recording
or
My Test 123.wav
These will not work..
These type will work ... for example
MyVoiceRecording
CharlieSaysHi
etc.
Hope that helps!