There's a few main reasons.

Firstly is other conflicts. At the time (or at least at points during the American Revolution) Britain was fighting on several fronts. India took the majority of the British troops as it was seen as being more valuable to the empire, and the was some fighting going on there. This also tied up a lot of logistics and expense, taking away from what could have been thrown at the US.

The most important foreign influence however came from France. From 1775 France had been supplying America with money, and in 1778 open war broke out between the UK and France (who was wanting revenge after the Seven Years War). Read here for some more information

With Spain joining in as well, this left Britain fighting the next two largest navies in the world in European waters as well as around the world, plus defending the areas of the empire now under threat from the combined French and Spanish forces, as well as the trouble in India.

This leads to the second point, money.

Britain had spent a lot in the Seven Years War, as well as lost a number of men, and these additional costs were a serious strain on the already depleted British war chest. A round trip to America could take over 5 months, supporting a large war over there indefinitely was financially ruinous, especially with all the other expenses with supporting the war in India and against the French and Spanish.

Thirdly we have public opinion, which had started to shift against a long war with America, making it even harder to justify politically.

So you're right that Britain never sent all it could have against the fledgling USA, but not without reason, and not without incurring such heavy penalties (financially, politically and in manpower) that it wouldn't have been worth it. It was a time when Britain was at war on three (maybe four depending on how you want to define it) fronts spanning more than half the globe, and giving ground on one in order to win the others was a sacrifice that was made.