If its a hardy one, put it in the ground - I had this trouble, nursed it tenderly in a pot, after 4 years, gave up and shoved it in the border, where it flowered beautifully for the next 4 years till I moved.

They need more light to bloom than most people think. Indoors, I would put it where it got several hours of direct sun, especially in the morning. Often they are happier as an indoor-outdoor plant.

Thanks - I'll plant it out then! Really I've got nothing to lose at this point. It's been in the hottest, sunny part of the garden which is usually too much for most things. It's so strange as the other one is thriving and is having it's best year ever. The rubbish one is sat next to it doing nothing!

When you tip it out to plant it, Joo, you may discover the problem. Sometimes they get this random root rot, and one plant will be affected, and the next won't be. If the roots are turning black and mushy, pull off all the affected roots, and the mushy stems and leaf bases, wash it well, and dust everything generously with cinnamon, to stop the fungus.