My Glenn and Carrie did the same but my Julie seedling and Madame Francis held a lot.

Did you have a pollination problem as well?

I had pollination problems so I went to the home depot and lowes garden centers and watched which flowering perennial plants bees hung out on and bought several.

Once they grew up and flowered in my "bee habitat" area of the yard bees started to come in large numbers. Once bees came then wasps and flies came too.

I bought basil, Mexican heather, lantana and a few more.

I also planted a carambola and guava which also attract bees.

You also could have a fertilizer or soil issue you can look into.

Mine is an "indoor" screened in area, so I am not sure about attracting bees naturally.

However, given a choice, what works better for indoor mango pollination- oscillating fans put next to the trees, or should I go ahead and buy bumblebees or lacewings and release them during flowering?

(only concern about bumblebees- I have little kids at home; hopefully no one gets stung!)

Sounds like a problem you will never get over unless you put them outside the screen when in flower.