Smokeys right. Bascally there is no point to have an image on both sides of a polygon, becuase it would basically double the cost on your computer to produce the model, and seeing how most players of your game will never see that side of the model is basically useless and a waste of important resources. So what this means is that if your polygon is facing the wrong way… When you look at your game you will see nothing.
Forunatly, because this problem is so common, blender has a feature that allows you to flip the ‘normals’/‘polygons’ view.
This video will hopefully help a bit https://www.youtube.com/watch?v=p9Q5c-4AeRA
PS: There is a way to tell blender not to show the other side of the model(so it will be the same in blender and in unreal), saddly I have forgeten how todo this, and I am on Holiday.(this will help show you the problem as it will show a massive hole in your model)