Textures suddenly gets streched when building the level

Hello everybody!

I am currently trying to get used to UE4. So I am very new to this engine.

I just created a new material with a 64x64 texture I imported. When I apply the material to my wall surfaces and build the level, some of the wall textures gets suddenly streched without me doing anything!

Take a look at the screenshot.

Does anyone know why this is the case?

Greetings from Germany!