i had posted concerns about drop rate on dodga/alvin / fixer grid quests.
further testing and information gathering reveals:
after spending too much time camping bots in one spot they stop dropping their items. leaving quest items in bodies seems to make this time extra short.
once mobs stop dropping items you need to move to another site to camp bots; and the cycle repeats.
if you are lvl 20, don't attack bots right next to trash king or his lackey.