I noticed something weird recently: when I load a saved game, my capital levies are lower than they were when I created that saved game file. Other counties are not affected and remain at full. It looks like what's happening is that for some reason the Studded Bastions (+200 archers per holding) on the Great Walls resets the archer portion of the levies to zero strength when the saved game is loaded. So during the course of play, the garrison will replenish to full (since they have archers too, even if we don't get the breakdown by type), then the archers in the levies will start to replenish, but they'll never get anywhere near full strength before I stop and create another saved game. Repeat endlessly, and the capital levies never come particularly near full strength anymore. Presumably the problem will get even worse once the Great Walls are larger and I build Soaring Fortresses for (nominally) +100 heavy infantry per holding.
This seems to completely torpedo the whole "Great Walls and lots of castles in your capital" plan.
Is there any workaround? Could something else be causing this problem? The fact that it only happens in the capital, and only zeros out the archer portion of the levies, strongly suggests that it is a problem with the Great Walls, though.