Rep Power: 0

Ok when I first got my car the compressor was bad so they replaced it. A few days after I was driving on the highway and when I started to slow down the belt started squeeling bad. Looked at it the next day and the bolt that goes through the alternator that hold the wingnut thats sticking out on the front had vanished. So I put the new bolt in there and tightened everything back up. A few weeks later the belt started squeeking again and I tightened the wingnut over 2 full turns to pull the alternater up some more. Now that was like 3 days ago and its squeeking again, am I missing something here? Why does the belt keep loosing tension?