Hi Peter, thanks for the prompt explanation.
Yes, I can see that the pressure is higher on left. Also the animation shows the moment causing the block to be pressed more on the left.
Can you help clarify a bit more as to what is causing the normal force (due to gravity) to be not uniform ?
Also, I increase the force so that it is greater than frictional force to see sliding behavior (in transient structural, as I understand that static structural is to be used when a=0 or net force is balanced). However, I experience error that abrupt change in contact status. Could it be because of the limited length of the lower long block and the top block flies off ?