Tailgating rookie

hey all, hoping someone can help. My neighbor gave me his round generic directv dish that he had just installed but didn't like it so I decided to try it out tailgating. I mounted it on a cement patio block, put some carpet under it and had it on top of my car to tailgate at Giants Stadium, East Rutherford, NJ. First time I tried it, I got the signal after 30 minutes and it seemed all was well. Second time was not successful. I got the signal on my hughes receiver but when I hit "watch tv" it just stayed on "updating guide". On the signal screen I also noticed "mismatch" message. How can I have a signal and it doesn't work? I was switching back & forth from 101 to 119. Does my satelite shaft the I bolted in have to be straight up and then I just adjust the tilt of the dish? Thanks for any help someone can offer. I think I just lucked out the first time, i'm confused. Any ideas?