Lamb wave + Abaqus + High Frequencies

Hi

I have prepared a very simple model in abaqus which is an Aluminum beam under A0 lamb wave. It wors perfectly with low frequencies andthe amplitude plot shows a clear one in various control points.

But when I increase the frequency to 200khz that nice and neat plot is not obtained. I have attached two plots 1-excitation wave 2- amplitude on control point

could you please advise as to why this happens on high frequencies and what is the solution to this ?