Hello and sorry to bother people for what im sure is a very stupid and quick answer, but i was working on trying to follow the video and got some odd errors. Hopefully someone can help me figure it out from the screenshots. I just made two quick floor meshes in Blender to try and follow the video, i figured out how to force it to export to a certain dimension (100,100,5) and double checked its size after import into UE4, so they both are those dimensions and should work if i understand how your using yours in the video, but i have a sneaking suspicion something is wrong with them anyway and thats whats causing the. Here are the screens:
https://goo.gl/photos/53njoC3mb33XaDDs7
https://goo.gl/photos/qozUVJgdr9Udt9uY7