You are forgetting several things.
Normandy was taken several times by each side. As well as Aquitania, Maine and Anjou. A region was not impregnable, and it became less impregnable when the use of canons spread in the 15th century. And in the mid-15th century, the french had the best canons and in greter number.
When, a few years after the treaty of Brétigny, hostilities resumed, it took very few man for the french to take back most of the territories ceded to Edward III. Defending territories is very costly because you need to defend each stronghold while the attacker only need to take them one by one.
Fundamentally, the balance of power was very much in favour of France when its armies were not led by incompetent idiots. It had much more ressources and fought on its soil.
As far as holding Normandy is concerned, the politics of Henry V's brothers was decisive in alienating the french population, not tle least in Normandy.
You are right in reminding that the french took almost 20 years to take back Normandy and Guyenne-Aquitaine. In fact, they took 10 years for Normandy and 10 more years for Aquitaine, but the delay for Aquitaine was deliberate.
Aquitaine had remained more closely linked to England than Normandy. So the french decided they needed to go slowly.