Is there some malus on effectiness on Japan Mainland ? I'm stacking units in order to prevent any allied landing, and the effectivness of units is going lower.. Even not moving.
Food is not a problem (japan is almost only 9 supply level).
I didn't though about weather, i understood the weather effectivness malus was a reduction of the increase. Not a drop. (hope you are understand, i'm not a native english speaker ^^). But maybe i missunderstood.
Post a screen shot so we can be sure everyone is talking about the same thing. Especially since you refer to "stacking".
Units lose effectiveness every time they make an action like moving, attacking, etc. They also lose effectiveness due to bad weather like rain or snow in hex. Effectiveness increases when during clear weather when no action taken. And, will eventually reach 100%.