In my current game, I did the invasion of usual Southeast Asian territories (DEI, Malaysia, Brunei, Philippines, etc) and also successfully invaded Hawaii. It took about until 42 and 43, but the US has thus far tried to retake Hawaii with a marine and army unit supported by several carriers and battleships, is currently running a marine unit around New Guinea (After I successfully seized it), and just launched a massive invasion of the DEI with two marine units, an army unit, and a tank unit. So I think as Hubert says, it takes time for the US AI to build up.
I have to say I am rather impressed with the AI as someone who is new to the game. The US counter invasion of Hawaii was well coordinated with sea and land units. The US briefly retook Honolulu before I was able to pull Japanese naval and marine units to push the US fleet back and retake Honolulu. For the invasion of the DEI, the US Transports went around the south of Australia thus avoiding the two subs I had around New Guinea as sentries which has caught me by surprise. I am now scrambling to get additional units to DEI before its too late. Could not come at a worse time because I just launched an invasion of Burma. Definitely going to be a close battle.
This is new since launch and glad to hear AI is at least attempting to re-take Hawaii. Still too easy for player to capture in first place - I have never taken Hawaii against MP player and never lost it either. Should be some supply malus or some such.