Our online group has had this problem too.
Sometimes they don't go off at all and sometimes the delay fails and they blow-up right under your tail with disappointingly predicable results.
1) there is a minimum drop height for different bombs - hit the TAB to get HUD info on the one you are dropping in game.
2) check your delay settings - check every time. Check again. Sometimes the ghost in the machines changes the settings on you.
3) under difficulty settings - weapons section, check the "bomb fuzes" check-button. Try it on, try it off.
4) some other forum post suggested that tests of the Russian bombs have shown that the time the fuse needs to arm the bombs can be influenced by the delay timer setting. If you want to drop bombs at very low altitude you have to select a delay of 5 seconds or more. This should solve the problems when doing low level attacks.
I hope this helps.
~Flatlander