Probably just too young; young trees often have incomplete flowers. Perhaps older trees do also; but there are so many flowers, they may go unnoticed.
BTW, no need for hand pollination or anything else to make lemons; the lemon flower is a perfect flower and makes fruit with no help from anyone or anything.

if you dont need to pollinate lemons. How about other citrus. Like grapefruit or mandarins? As mine are flowering now. Thanks

There are a few species that need pollination from other varieties; but almost all citrus are perfect flowers. If you google your specific variety you should be able to find if it needs "help". Note: If you want to make seeds, then you need bees or other help.

I have three indoor meyer lemons. This pic is the aphids on one right after being sprayed with Bonide. I brought them in at the end of summer and they now for winter live a few feet under my T5 lights in the living room, which they seem to like quite well. They are all a foot or so high and probably 1-3 years old. I have pruned some but not lots. They've never had actual lemons. I bought them this spring and they easily doubled or more over summer. One of their siblings in my office at work has one big lemon but has never flowered at all on my watch so far,oddly.

The three at home have been flowering for a few months; one seems to have finished and the other two have many clusters. Leaves are fairly healthy; medium to dark green; a few have those twisty-curly spots and bumps but not most. However, almost all the flowers fall off, often before they even open. It seems these should be the precursors of lemons, and they are all falling off!! ??? why are all the flowers and buds falling?

As a separate issue, I also keep getting aphids; clusters appear on the flowers and then I spray them and wipe off and they return later. I say separate because even the buds with no aphids all all are falling. And I've also got what I think are intermittent fungus gnats - multiple tiny little guys in the soil and edges of pot. I have tried using Neem spray, on the soil and tried letting it "really dry out" before watering. Also tried a nastier spray a while back. Nothing totally clears them although the gnats aren't too bad right now but keep resurrecting; i know that's partly their life cycle but i have taken most recommended steps. I can't let the lemons totally dry out I'm already worried about the roots due to the gnats. Yikes, so many issues.