It's harder than you think
Sophie had great difficulty finding a unit of repeat for a blue-blue-pink train of cubes. This was her first attempt.
After some help from her teacher and several attempts, she eventually managed to break the pattern into three identical units.
After several more examples like this, Sophie was able to confidently find a unit of repeat in complex repeating patterns of cubes. She was also able to draw these patterns and create her own repeating patterns using other materials.