This is guessing only, at this point: my Allium lives in a pot and I haven't refreshed it for at least two seasons now. It has produced multiple grass-like offsets in that time. The flower stalks have reduced in size and the display is less than best.

Part of that could be hunger (new medium needed asap). And part could be that the bulbs are too crowded/successful to reach flowering size.

I'm going to treat mine as I do my Muscari - wait for dormancy and sort to size. Plant up the largest bulbs and grow on some of the smaller ones.

I do notice that where the Allium has seeded (in the middle of the rockery, of course!) the leaves are taller and the flowering is good. The leaner soil/grit combination seems to have suited it. I'll be watching for the emergence of offsets. I like it, as a plant. Just, Not There, as any sort of clump.