Farming Midgets in the Preserve- Did I goof this up?

I'm playing through UVHM for my first time, and recently-arrived at the Wildlife Exploitation Preserve. A couple of weeks ago, some friends showed me how they were "farming" the 4 cardboard boxes for loot midgets/pearlescent weapons/legendaries/relics.

*** For those of you who don't know of which boxes I speak, they are located in a small room on the right-hand side, immediately before you reach the (what turns-out to be) empty holding pen for bloodwing. (you later find that bloodwing is moved to some place more "dramatic.")****

ANYHOW. . .

I am aware that, in order to have the midgets spawn in those boxes, you must have accepted the "Doctor's Orders," mission and NOT have picked-up the set of notes that appear in those boxes.
I have adhered to this stipulation, and managed to successfully complete maybe 3 or 4 runs where each box would spawn a midget. This was yesterday morning. When I went back last night to do some more farming, 3 consecutive farming runs yielded NO midgets at all. I then decided that it wasn't worth wasting my $$$ and ammo anymore, so, out of rage, I proceeded with the story-line mission. This ended in failure, as I wasn't able to take-down bloodwing solo. Rage-quit, and that's where I stand now.

Does anyone have any theories as to why the midgets didn't spawn? I have two:
1- Did Gearbox set-up some sort of daily-maximum number of midget spawns?
2- Do I have to complete the storyline mission first?

If it makes a difference, I noticed that the midgets spawned when the optional (collect 10 slag samples) objective had not yet been completed. However, once I had collected the 10 samples, they stopped spawning.

Have I completely screwed myself, somehow, out of being able to farm these boxes? or is there somthing else I'm missing?

Thanks, friends!