My delks mk1s are the same. But just looking at how you've set them the sensitivity is not the same on both alarms. The left hand looks set to a lower number than the right hand rod. I've found the tiniest adjustment can be the difference between sensitivity.
Also when you switch them on do you set them on HI or LOW? On HI your alarm will register the tiny knocks however on LOW it won't register them. I tend to have mine on LOW when it's windy or fishing over lots of particle and small fish are present.
Hope that clarifies a few bits for you. If everything is set the same and it's still a problem then maybe the Y bar is knackered. I had this problem and just sent off to delkim to get it replaced. Not costly at all.