Using Monte Carlo in bertool the plotted Ber curve is coming as a horizontal line.Why is this happening?

In my ofdm simulnk model,when I am plotting the BER vs Eb/No graph using Monte Carlo of bertool, I get a horizontal line.While computing the BER for various constant Eb/No i do get proper values on the display.I have followed all the steps mentioned in the simulink example explaining bertool in the document BER in Mathworks.Why am I not getting the waterfall curve?

[Amee Badiani]

I am facing the same issue. Pls suggest me what to do...