Komori ink density differs gripper to tail

We have two 2007 Komori LSX a 29" and a 40". Both of these presses show different ink density from gripper to tail. When we run multiple-up of the same image and it is cut down the shade differs from pile to pile. the 40" runs conventional inks, the 29" runs UV inks. Someone said to have the operator adjust roller timing. I don't know what he is talking about and I am not an operator. Can someone explain in layman's language what is happening?

Thanks

Slice.