No, we don't do anything and ours are OK. If yours are performing poorly, as they seem to be, give them a feed after they have finished flowering. I would give a handful of bone meal or a few liquid feeds of tomato fertiliser. There are other fertilisers available from the garden centres.

I was curious to see what the answers would be too. My alliums too, are smaller than those from last year (but I think there's more of them, so maybe they concentrated more on producing baby plants and not so much on having large flowers).
Thanks for the tip Bublaholic, I'll give mine a good splash of tomato fertiliser once they're done flowering.

ok thanks for that I must admit i only feed my garden soil once in spring and never really put much fishblood and bone on but there is a rambling rose in the same border. Why would I wait till the allium as flowered befor using tom food please?