How aggressively are you lend-leasing China?
Balancing the Sino-japanese war has been difficult in HOI4 and the game more or less ends up each update with a fragile equilibrium where China needs to be weak enough to be pushed away from the coast but strong enough to hold out from there. This is challenging because losing industrial capacity from losing the coast should cause a downward spiral, but Japan needs something to hold them back from snowballing into China. Currently, China gets a lot of buffs when Japan initially invades (being able to access their national ideas giving them defensive bonuses faster than Japan gets their attack bonuses), but AI China depends a lot on foreign support (like historically) to keep Japan at bay.
Unfortunately, HOI4 struggles to model the logistical challenges that kept Japan from advancing inland, and while the resistance system in Husky was able to improve upon the challenges Japan had in forming a collaborationist government and keeping insurgents out, it's not enough to completely slow down the Japanese. Furthermore, there is the issue of overbuffing China, which can cause Japan to be thrown into the sea by the Ai fairly early in some circumstances.
If you're starting in the 1939 start you're basically out of luck - the devs generally regret adding that start-date and it gets a lot less attention by the devs and the playerbase than 1936. As Britain you can't do much besides lendlease China (if you have the dlc, otherwise you can only really send volunteers until WW2) starts. I personally believe the oil embargo needs to be added into the game as an event for the US and UK to set up the Pacific theater.