Please excuse the fact that I'm writing from mobile, and that I'm admittedly more familiar with the effectiveness of larger bombers. I also apologize for not having raw numbers at hand, but these can be troublesome on their own (for one, they're rarely accurate, you might have multiple gunners from multiple aircraft all claiming the same kill, and thus you might see them claiming 4 downed fighters when only one, or even zero, actually being shot down!) As such, for the moment I'll try and illustrate the problems at hand to show why they weren't very effective at actually shooting down attacking fighters.

For the most part, defensive armament on light/torpedo/dive/etc bombers was mostly just a deterrent. One of the biggest problems was that they most often employed smaller caliber weapons such as 7.92mm/ .30/.303 caliber machine guns. By the time of WWII, most fighters were armored well enough that such small caliber weapons were of little effect, though of course they weren't impervious to them. These lighter machine guns simply didn't have much stopping power, and were out ranged by the heavier weapons employed by fighters of the day.

Another problem is the added weight and drag that comes with employing such armaments. Such factors reduced range, speed, maneuverability, and basically every performance metric out there.

Yet another problem being the fact that these guns were often only mounted in such a way to provide coverage to the rear quadrant of the aircraft, and often leaving the aircraft vulnerable to attacks from below.

But wait, there's more! If you're one of the first 1,000 callers, we'll throw in the difficulty of actually aiming one of these defensive guns, especially against targets that weren't flying in from straight behind your aircraft! Trying to actually hit a moving target is maneuvering and not perfectly lined up with you is insanely difficult. Indeed, during WWI it was quickly found that the easiest way to aim a gun was to mount it pointing it straight ahead, using the aircraft itself to aim. Early fighters often had turrets mounted behind the pilot, but these proved so hard to aim that they were generally dropped in favor of single seat fighters with forward firing guns (not just because of the aiming issue though, also the other factors mentioned such as weight, etc). Think about how hard it would be to hit a target at a firing range when you have a 90+ mph crosswind, and you can start to understand just one factor in how hard it can be to aim such weapons. If you've ever played a FPS game where there's a level wherein you're a passenger on a moving vehicle being driven by another player or the AI, you can begin to understand another difficulty in aiming such a weapon - it can be very difficult to properly track a target when the platform you're on is moving in ways that you can't perfectly predict.

We have many accounts of such aircraft performing missions in which nearly every, and in some cases, every single, aircraft was lost. Not always to enemy fighters, but sometimes also ground fire. And it may not seem obvious at first, but while the turrets might be used against enemy aircraft to interfere with their attacks against you, they do nothing for the threat of ground fire. Yet the performance losses from the addition of defensive armament can still make you more vulnerable to said ground fire in that the aircraft will be slower, less maneuverable, etc.

But because of their vulnerability to fighters, we often see the armament being omitted from aircraft originally so equipped, either in the field or in factory produced variants, in an effort to improve performance. And taking it a step further, we sometimes see such aircraft being removed the fight altogether. The Stuka for example, upon its first encounter with a well prepared air force capable of going toe to toe with the Luftwaffe, was withdrawn from the Battle of Britain due to its vulnerability and the losses suffered. While the deficiencies of the Stuka were not all the result of the defensive armament, it wasn't enough to overcome these deficiencies that made them so vulnerable to enemy aircraft. Indeed, the USAAF and the RAF in particular focused more on adapting fighters to such roles, and with the Luftwaffe phasing out the Stuka with the ground attack F variant of the Fw-190 more and more as the war goes on.

Now all that said, they could still make a pilot think twice about simply coming up from behind and casually engaging the aircraft. And the easiest way to shoot down another aircraft is from directly behind, so they at least made life more difficult for intercepting fighters.