Dissapearing floor

[lboobyjonees]

I was making my first mod and when I went to test it out some parts of the floor were missing they were appearing fine in the GECK.

 

Some help telling me what happened and how to fix it if it happens in the future would be nice, thanks.

 

http://i389.photobucket.com/albums/oo334/Omgitsamonkey2008/mod.png