Maximum Likelihood Classification not working correctly in ArcMap

I am trying to classify the 3 bands RGB image in ArcMap 10.8. I have prepared some training samples, and performed the MLC. The result is returning 3 classes instead of all 6 classes that the training samples have.

As I completed classification on another 4 bands RGB image, and I had similar number of training samples, image was the same size, I have used the same algorithm, and everything worked well, now I have no idea why is that no longer working on the 3 bands image.

Of course I have generated new signature file for new image, and set the right input.

What could be the cause of my problem?