For starters, 160 is pretty high for a mash. Its possible that in heating it that high, you've denatured a decent amount of the enzymes, which would prevent them from generating sugars. Secondly, how much water are you using, and how long did the mash go for? I'll admit that I don't normally taste my mashes, so I'm not sure how sweet they should taste. However, I could imagine that some of the bready flavors you get from Munich being roasted a little more than Pilsner might wind up covering up some of the sweetness of the sugars that have been released.