Enemy troops loot my provinces all the time, but my never seem to, even when I leave a big stack in an enemy province for months, how come?
A province is "lootable" if it is all of the following:
Is an enemy owned and enemy controlled territory
Does not have the "looted" modifier (which lasts for 6 months)
The province does not border any province where the looter has military access (allies, agreements, a province occupied by the looter or an ally).
If you are fighting a war in the HRE area with or against the Emperor (or you are the Emperor) you have 'military access' to every HRE-enabled province
perhaps this is your issue?
Look for a coin floating up at the end of the month. I do not get a modifier on the province for looting, but I do see a coin amount floating up at months end.