Answer: Flowering is dependent on plant maturity, not on size. So, even though all your plants are about the same size, two of them were older and therefore able to flower this past summer. Give your plants another year and they'll catch up. As they mature, they will produce multiple plumes each summer. They grow and flower best in full sunshine so make sure each is getting at least 8 hours of sunlight during the spring and summer months.
Best wishes with your landscape!
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