The type of lnb will help also? Is it an lnb or is it an lnbf? If it's an lnb, then it could be a bad servo motor (polarotor). It could be something with the wiring. You can test the servo motor by simply turning the lnb 90 degrees inside of the feedhorn assembly. An lnb will change polarity if you turn it 90 degrees. An lnbf will not do this. If it's an lnbf, it could be you have your receiver set-up wrong in the menu? What kind of receiver do you have? The kind of dish will only matter if it's too small. For C-band you need at least a 6' dish, I would recommend at least a 10' dish.
Al