No flowers on my Agapanthus

Hi All

I've got several agapanthus plants, all in tubs (so they can be brought in during the winter), they're all well established and have flowered every year. Until this year that is, all my 'blue' plants have flowered just fine except my show-piece 'white' agapatha has refused to send up stems, just lots of plane-old-leaves. I'm not sure which variety it is as it was given to me during a tidy up. But it has always sent up about 7~8 stems about 4ft long on each.

Is there any reason it wouldn't have even tried this year ??

Any advise would be appreciated

Regards
John