I've had a lot of experience with the 101.0W AMC-4 sat and I have locked on the 12120 TP with a 20" DishNet 500, some receivers have weak tuners and even with a 90cm perfectly aimed, I can hardly get 12120 with my Manhattan receiver but I can pop right in with my Captive Works because it has a sensative tuner.
Another thing, make sure you skew to accuracy when tuning the dish, The GloryStar instructions say to aim in the center of the sats 97.oW and 101.oW but you may consider tuning the set up with just one LNBF on 101.oW, set to TP 12120 and tune your best, then hook up to the 97.oW lnbf and see what you have.
I personally don't care much for the 12120 channel and if I can't lock onto it, I don't lose any sleep, and I could care less for the 97.oW programming. The best Networks to watch are TP's 11822 & 11983, I'm sure your customers will agree once they see for themselves.
They may just have to buy a new receiver with a stronger tuner as I did.
Good luck