Hi Rishi,
From the figures you've provided, it looks like the BER is quite low for SNR values of 0 and 1 dB, falling below . Since logarithmic graphs can't plot a BER of 0, it's possible that the BER value for higher SNR values is zero, which is why it is not showing up on the plot.
To get a more accurate BER reading for higher SNR values, we could try increasing the number of samples in the simulation. Another approach we can try is to run the simulation multiple times and then average the results to smooth out any anomalies that might occur especially at higher values of SNR where BER is probably very low.
Hope this helps.