A big issue was really the lack of manpower. Or at least manpower that was mobile and able to go on the offensive.

The British defense in Canada was quite strong because of domestically available militia and an alliance with the local native Americans but even than they were often outnumbered.

At the surprise British siege of Detroit, the British were actually outnumbered 2/1 and only succeeded due to an inept American response and a very effective usage of trickery.

The British also lacked an ambitious leadership and plan following the death of Sir Issac Brock at the battle of Queenston Heights. This isn't really to say the British lacked effective leadership as they coordinated the war quite well. However many British Officers simply saw defending Canada as the objective, rather than pursue a wholesale conflict with the United States. By all accounts that is an exceptionally prudent idea as that would've engulfed the British into two high intensity wars.

Both sides crippled each other at the right time. America was initially surprised and than badly demoralized by continual defeats by the British. Often in a resounding fashion; equally effective was the death of Sir Issac Brock who provided the most significant vision and initiative in pursuing the war against the United States. He was the architect of pushing the Americans away when they attacked and he likewise would've been the man to keep the fragile coalition of Natives and British together on a unified front.

After Brocks death the front somewhat stagnated. Both sides had very little political capital to use in the war and until 1814, there was no tangible British plans to counter attack. When they began to be implemented the conflict had simply ended by than.