Kindred does not always gain stacks while killing a marked scuttle crab [GAMEPLAY]

When kindred kills the scuttle crab that has her mark on it she does not always get a stack.

I have played kindred adc a few times and when I kill the scuttle crab that has my mark on it I do not always get a stack. In one game i killed 5 scuttle crabs that had my mark on it and only received one stack.