Based on the pictures I would agree that the skew angle is not set correctly, but above all I would not really trust that adjustable LNB arm either. It seems that not only the angle is variable but also the height? (since it uses a longitudinal hole) ?
Definitievely worth tweaking after you made sure that the azimuth is 100% corrently set to maximum signal.
When using the SDR in wideband mode (e.g. displaying around 1 - 1.5 MHz bandwith of spectrum) you should clearly see with your dish size at least 1-3dB noise level incease in the middle of the passband as compared to the noise levels seen on the spectrum display outside of the passband)