Bombs need to travel a certain distance in the air before they become armed. This distance can be achieved vertically or horizontally, or any combination of the two.
Are you sure your bombs are dropping and not exploding? If so, you need more altitude before you drop. Or, if you are doing a "loft" bomb approach, you need to increase your drop angle and/or distance from target at drop.
There is the possibility that your bombs are indeed dropping, are indeed arming, and you are just not noticing their impact. The AH default explosion noise is entirely underwhelming, and the graphics effects are something you might find in a 1998 copy of European Air War. If you are used to newer games, these explosions will probably not register with you. Also, if you do not destroy an object, you will not get a "XXXX Destroyed" message, and this may lead you to think you've entirely missed your target, when in fact you've damaged it severely but not entirely destroyed it.
Hope this helps.