Mirrormod needs to be aplied for new mirror to work.
At the included video time when mirroring the meshes, my middle one needs to have its mirror applied. If not i will only mirror half of the middle part. Since the middle part is also mirrored, it probably chooses the part with the axctual geometry, and not the mirrored part. But Kent doesnt apply it in the tutorial so i wonder why i had to. Can this have to do with origins?