drop rate lower over time?

I notice after farming the same 3-6 monsters over and over that after the 3rd time, their drops reduce by half, and then after 4th time, their drops reduce further to like 80% or so until almost nothing appears,

Is it just me or is that really happening?

It's called the Farming code. You can't farm the same place over and over. If you do the code will kick in. Only way to get rid of this is to do missions and quests.

Just farm mulitple locations....

build tabs ftw in this case. farm 1 spot, load next build and go on.