Comparing the defeat of the Luftwaffe over Britain in 1940 to the victory over Germany in 1944 is interesting, but cannot be put down to any singlefactor. There are a myriad of reasons for the two outcomes
Here are some of the factors as I see them
1) Lack Of Reserves...relatively speaking, the Luftwaffe enjoyed a greater advantage in front line forces relative their enemy in 1940 than the Allies enjoyed over Germany in 1944. But they lacked an effective reserve, which meant every plane lost or damaged was much harder to replace.l In both 1940 and 1944, the Allies enjoyed a huge superiority in the available reserves, which meant that pilots could be turned around even when their mount was lost or damaged
On a similar vein, the Germans never had an adequate pilot training program. They just could not replace losses fast enough, and through the war tried to counter this by cutting back on training hours. This proved a disaster for the Luftwaffe and Germany. But it was a problem not easily addressed....the Germans never invested in support services nearly as heavily as the Allies, because they could not afford to, and lacked the oil to put a large force for training into place anyway
2) A failure to appreciate the effects of British Radar defences. By comparison the allies appreciated the effects of German radar and took elaborate steps to lessen irts effectiveness
3) A failure to concentrate forces. Typically the Germans attacked with strikes of around 200 aircraft each with a bombload significantly less than a B-17 or B-24. By comparison, it was not that uncommon for the allies to attack with a thousand bombers or more. Attacking in large formations reduces the effects of the defending fighters and makes the strike easier to defend. In the BoB the Germans attacked with multiple strike, using a fighter force inadequate to the task (should have been at least three times larger than it was).
4) Bombers that could not defend themselves properly. Whilst the US heavy bombers proved unable to fly completely unescorted, they nevertheless were blessed with formidable defences and were hard to shoot down.
5) A lack of clear strategy, and a strategy that was unrealistic to the capabilities of the force being engaged. In the beginning of the BoB the Germans set out to destroy the RAF as a fighting force. I dont believe the Luftwaffe had the strength or the means to do this. Even in its darkest days the RAF always had the option of pulling back out of range of the 109s and using about 3-4 days to recover. Given that Sealion was not ready (in a loose sense) until late October, there was no real threat arising if the british did withdraw. Its one of those common furphys that the germans had the RAF in its sights, but was denied by Gorings incompetence. I actually think Goring was ahead of the game, By September, only one of the British airfields had been knocked out, and none of the radar stations. The British were losing fighters, but they were replacing them much faster than the Germans were replacing their losses.
Attacking British cities would have reqiuired a heavy bomber force, which the Germans simply did not have.
By comparison, the Germans in 1944 could not retreat, due to the long range of the Allied air force. They were losing 1000 aircrew per month and could do nothing about it
6) Limitations of the aircraft. Ive already alluded to the shortcomings of the German Bombers. Their fighters also had limitations. The Bf 110 could not complete its allotted role of long range escort, and the 109 was too short ranged to do the job in its place. by comparison the Mustang in 1944 could excort Bombers allover Europe and fight competitively when it got there. There are few other aircraft that can claim that in this period.
Lots of other reasons....but I have to mow the lawn, or risk crashing and burning myself.....