#1: two wires were placed the wrong way.. Luckily they weren't the timer ones, or else the bomb would have blew up before they were at their destination
#2: fuel... During the flight and returning home, the crews had trouble transporting their extra fuel to their main fuel tank, which led to them running out of fuel very soon. (and yes... they were trying to fix the problem while dropping the bomb)
hope this helps :D
