And I agree on not having them potbound, in my experience, they bloom perfectly well grown in the open ground (provided they're a hardy variety), in fact, rather better than in pots. If yours are in pots because they're tender, pot them on into larger containers and really, in pots they need feeding. If they're not a tender variety, plant them out.

I have one that was pot bound and some of it died over winter. I repotted it in a largish pot with a geranium for company. It grew like mad. Made three buds and flowered. I think it went yippee I'm free. I bet it doesn't flower next time round though!

I don't know about he hardy kinds, but I do know that the tender ones like a fast-draining soil to bloom well.