The reason only one F-117A was shot down was because the USAF got arrogant, they flew the F-117 into the same target area three nights in a row! The Serbian AD noticed that targets in this area were being hit precisely at a few minutes after 10PM local time. Studying the the targets in the area and the placement of the radars they noticed there was one area where the gap being two fixed radar sites was greater than in other areas. So on the fourth day, the Serbian AD placed a mobile radar in the gap. A few minutes before 10PM the mobile site turned its radar. A few minutes later the F-117 flew almost directly over head, killing the F-117 was easy after detection.
The USAF saw their mistake and never flew the same routes in and out of target areas again. The F-117 was shot down on day four of a seventy-three day campaign. The fact that F-117's flew continuously during this period of time, shows the killing of the one F-117 was NOT the result of some technical breakthrough in detection of stealth.