Hi everyone. I'm new to Maya and I'm trying out modelling on my own. So I'm using this e-book which describes how to model a character from that Delgo film. I got to a point where I had to use a mirror plane to create a mirrored instance of the left side of the geometry so it formed a full geometry, but when I did that, some faces mysteriously vanished (like shown in the picture). Please what is the problem? Did I do something wrong? How can I correct this? Thank you.

This is a screencap of the problem: https://www.flickr.com/photos/67988435@N04/8637575658/


Is there anything that can be done to get the geometry to be normal (full) again?