My feature map value is zero. How can I fix this problem?

Hello.

I’m using CNN for learning the shape features from the mechanical part images (Like levers, screws)

I made a lot of images that has lever, spring, cam by using matlab.

and I successfully trained the CNN model that has 97% accuracy for test dataset.

But last convolutional layer has the 4 output(4 Feature Map) and 3 Feature Maps are zero (like null space)
Only 1 feature map has values that activated with images

Why do the feature maps that are only zero values happen?

How can I fix the problem so that has 4 Feature Maps that has some values??