Maybe they're pot bound. Have you tried thinning them out?

Hi, welcome to GoY, lily-of -the -valley, or Convallaria majalis, doesn't adapt well to being grown in a pot, particularly on a balcony, it prefers a leafy, fertile, humus-rich, moist soil in full or partial shade, you can pot some up in autumn, and either gently force them, or allow to grow at their own pace, to have a fragrant indoor display, but they should be replanted outdoors after flowering, Derek.

They are fussy plants about where they will flourish - in some gardens they spread like mad and in others they won't grow at all. Mine are flourishing in a north faring very dry hard bed - moist and fertile it is not. My mother used to have a bed of them right up against the house wall, dry north facing and and draughty and they flowered well. Who can understand this?

Hi Sue, another example of plants being unable to read, lol, Derek.

Yes its a sad reflection on standards of plant education...

LOL, Derek.