Answer: There are several possible reasons why they did not rebloom. Most alliums would really prefer more sun, all day long if possible. If they are getting at least six hours a day including noon, that is probably enough though.
Next, you want to deadhead or remove the flowers and flower stems once the blooms fade. Allowing the plant to set seed pulls energy away from the bulbs.
The foliage helps rebuild the bulbs, so be sure to leave the foliage in place until it fades naturally, then you may remove that as well.
You could try applying a bulb fertilizer in fall per label instructions, and you could use a bit of complete fertilizer such as a granular 10-10-10 in early spring as well just to help spur fall rooting and then seasonal foliage growth. Good luck with your alliums!
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