Why do some faces become transparent?

I am a beginner,I encountered a problem when I create a warehouse model with constructor(the size of the warehouse is: width 100m,length 180m,height 10m).In the constructor, I add textures for the warehouse and it appears to be perfect,but when I export it and add it to my game which made with TGEA,there are some faces which I have added textrue become transparent.why?Anyone can tell my why?Thanks very much.