You 'just' have to be very accurate with all the parameters. Dive angle, release height and a release speed being the most important ones. Deviate with one of those and the low drag bombs fall short/long. Add to all this, wind. Does not take much wind to alter their flight path, be it crosswind or head / tail wind.
Since wind is such a big factor, then I note down wind direction and speed, and when preparing for my delivery or deliveries, I set my HSI course needle to wind direction. Gives me a better visual on how much I have to compensate for wind. With about 3-4 kts crosswind already, its enough at a 2000 feet release, to push the bombs off target by about 30 feet.