First of all, if you have Together for Victory and haven't done so already, check the combat log of the relevant theater to see whether the losses are really primarily from attrition and not combat.
Secondly, you need to pay close attention to attrition. Mountains, marshes, deep snow, etc. cause attrition for every hour you move through them and at the same time they slow your divisions down. Low supply can be another major problem because it affects all units in an entire supply area.
For perspective, a military factory produces roughly 10 infantry equipment per day and an infantry division has about 1000 of them. Ten infantry divisions marching for a day through (or attacking into) bad terrain/weather (~50% attrition)
statistically lose 10 * 1000 * 12% * 50% * 10% =
60 infantry equipment, i.e. keeping six factories busy just with their attrition.