There are 2 major factors in bombing, speed and altitude, the higher your altitude and the faster your speed equals a much narrower "cone of error" if you know what I mean. Think of it this way, I usually bomb with B17's at about 15k and my speed usually is around 230mph. Now at this alt and speed I have a little bit of an error margin where if my speed isn't spot on I can usually still hit my target. On the flip side the B29 is usually flown at 30k and at speeds of 300+mph, this means the error margin is is incredibly small, you have to have your speed and drift set perfectly or else you'll miss.
And one other thing to remember, the game has random bomb drift modeled so no matter how well you're setup your bombs will still drift. At higher alts this means they drift quite a bit.