The satellite positions appear to the dish to be closer together near the ends of the arc and the resolution of the motor makes precise landing more difficult. As the look angle nears the horizon terrestrial noise increases and clouds stack up and attenuate. Precise aiming is often required to pull these signals out of the noise.
Motors are typically designed to cover +/- 70 degrees from true South. Beyond these operational points, motors do not always correctly track the arc due to mechanical variances and installation issues.
I have always preferred a fixed dish for working satellites near the horizon as they provide more reliable reception.
interesting....i have never had any problems near the ends of the arc....