It really looks like Wall Lake 155.70000c 156.01500 157.41250c 159.61500 159.85500 160.15500
Or Sioux Falls 157.27500 157.48500 157.86000c 157.98000 158.05500c 158.11500 158.28000 158.55000c 158.59500 should make it to Canton
I guess I'm not sure what kind of scanner you have but I have a PRO-197 that allows me to store thirty some odd control frequencies, so I can basically get state radio anywhere I travel.
The c on the frequencies above denotes a control channel.
I imagine that you already have these frequencies programmed but it's worth a shot.
It's all here:
South Dakota State Radio System Trunking System, Statewide, South Dakota - Scanner Frequencies
But I'm sure you knew that as well.
I wonder if the problem may lie in your antenna system? Are you mobile? And what kind of radio are you using?
Any more info you can provide would be a great help to others who may be reading this thread.