Then as now, Manchuria is a grey zone where Chinese sovereignty is concerned, given that it only truly became part of China under the Qing. And the Japanese knew it, which was why they managed to get away with establishing Manchukuo in '32 and snubbing the League over the issue. China was different: both the IJA and the IJN knew the Americans were very sensitive of the Open Door Policy, and despite the heavy inter-service rivalry both branches of the military didn't want a war with America.
And starting in '36, they saw the advantage in America's growing apprehension over China's increasing militarization as giving Japan an opportunity to mend fences bruised by the Manchurian Intervention (which in turn allowed the Japanese a semi-plausible excuse to increase their standing forces to maintain a seven to ten ratio with the Chinese). And of course, Chiang's ambitious naval construction program starting in 1936 especially worried the Americans. Much like they later realized with the Soviets, Japan while perfectly capable of becoming the regional leader was nowhere near an existential threat to America. China (and later the Soviets) were both large countries with manpower, resources, and industry to match America - and therefore existential threats that Japan could never be.
Worse, Japan might have been arch-conservative in the vein of Imperial Germany, but it was a multi-party parliamentary democracy. Not so with Chiang's China, which was a single-party state reminiscent of the Nazis but lacking their racial superiority and such overtones. And in the Soviets' case, well, at at least Japan was free-market, which the Soviets were not until Khrushchev came along.