it could be that the others are yet to put up their flowers. mine don't usually flower until late july early august. They are heavy 'feeders' so make sure they have fertile soil and make sure that you give them a good feed say from august onwards to encourage a good corm formation for next year and water if they have a dry spell.
lack of water early on in the growing season may be a reason. or lack of water/feed when they were storing food last year.
How much sun do they get? they flower better in full sun.

Mine are showing no indication of flowering at the moment. I am feeding mine but maybe not as much as I should do. Mine were bought this year though so maybe the ones I have just didn't have the right feed/water before they went on the shelves. As Seaburngirl says, give them plenty of care and they should reward you with flowers next year.

Thank you both for your advice, mine are all at least one or two years old but as I've had bad health a lot this year, I certainly didn't start watering or feeding the garden earlier in the year, so that may be the reason, as you say, that they are not showing signs of flower spikes. I am now feeding and watering, so will have to wait and see. Many thanks again