S!
Do you have visual effects on low? On an earlier beta I had crap fps so I put it on low, until I started to wonder why aren't my bombs exploding. They were exploding, I just couldn't see it because of the stupid low settings don't show any kind of explosion
You can select the wanted fuse for each type of bomb in the loadout screen. Load the bombs in the left most tab using the drop down menus. These are the bombs that actually make into your plane.
Then select the Bombs tab and then the bomb type you are going to use, then you can select the fuse type from the drop menu on the center of the screen. Low level is the choice for any bombing below 1km, and the high alt for anything above that. Note: making any selections on the Bombs tab won't do any changes in your selected loadout, just on the bomb fuses.
Low level has a low arming time (the bomb must fly a short time before it is armed) of something like a fraction of a second, but a long delay. So it will explode after 14s from impact. High alt fuse is the opposite, having a long arming time, it must fly a long time before hitting anything to arm, so it is useful only for high alt level bombing. But after it has armed it will detonate immediately upon impact. Difference in damage is that low level burrows itself to the ground or ship or whatever it hits, high alt doesn't. Low level for such is good against ships as it will penetrate and then later explode inside. High alt for soft targets above ground because of the more "daisy cutter" effect as it detonated on ground level, not under. But in general, low level is good for most operations.