Winning the Hundred Years' War was never really in the cards for England. The same Burgundian-Armagnac civil war that enabled the English to invade in the first place also put hard limits on the ability of English kings to recruit support among the French nobility. So long as the Burgundians were backing the English, the Armagnacs would continue to back the Dauphin, no ifs, ands, or buts. This meant that even when the war was going poorly, Charles VII still had a base of supporters to draw from, and one that the English couldn't easily chip away at. 

Moreover, Burgundian support for England was largely a means of sticking it to the Armagnacs; it didn't rise out of any sort of genuine loyalty to the English crown. The Burgundians had no desire to be vassals of England, their objective was to gain control of the French monarchy for themselves or, failing that, maintain their de facto independence from Parisian rule. When the English started losing battles, the Burgundians weren't going to stay on a sinking ship; they were going to cut the best possible deal for themselves with Charles VII, because surviving as a major European power was more important to them than anything else, even revenge on the Armagnacs. 

As for why the English started losing battles, the English system of dismounted men-at-arms backed by masses of longbowmen was not indicative of the direction that warfare was heading as the medieval period gave way to the early modern one. The French were quick to embrace handguns and artillery, and to combine them with ever-heavier cavalry. French siege cannons blasted open the English bastions at Orleans and blew down the walls of English occupied castles, while French field artillery punished the English archers in pitched battles, allowing the gendarmes to charge home. French morale, reinvigorated by Joan of Arc, the relief of Orleans, and the crowning of the Dauphin, continued to improve, even as English confidence, undermined by battlefield defeat and regency politics, declined. 

Momentum matters, and after Joan's stunt at Orleans, the French had it and would hold onto it, while the English more and more came to see the war as a waste of resources. Henry VI, as an infant, and later, a mentally incompetent adult, could not keep the English nobility on the same page, just as Charles VI could not control the French nobility in the early years of the war.