Recalling the Treaty of Windsor of 1386, it seems strange to me that this is a gap in an otherwise consistent Anglo-Portuguese alliance during the colonial era. (bit.do/4zLQ).
The British were then a great power and Portugal could have gained territory/resources vis-à-vis Spain's involvement in the American cause.
In 1755, Lisbon, Portugal's capital, was vastly destroyed by an Earthquake. Then the Seven Years War started; Spain invaded Portugal three things times in a row between 1762 and 1763.
So when the American revolution began Portugal was still recovering.
With Portugal in the field these could be even a bigger trouble for England instead of a help. Spain would probably invade Portugal, which couldn't defense themselves from another Spanish attack in such a short time, and Spain would then not only control the mainland, Portugal, but also all his colonies, Spain would be ruler of almost 100% of South America, which would make it easier for them to attack the Brits on the other half of America, they would've an easier access to African Slaves due to Portuguese colonies in Africa which at the time would mean "more free workers to your empire" and off course the Portuguese colonies in India and others points of Southeast Asian and that would mine more species, more resources, more money...to finance the war against England.
Portugal not "helping" in the American Revolution was the better thing to do both for England and Portugal.