I'd suspect too much water. Even here, in dry California, root rot is a problem... I mix LEAST 1/4 of the potting soil with 3/4 perlite (or 1:4 ratio). Then I have to check to see if the soil is dry before watering. I think in Africa, they grow in VERY VERY porous soil. It's too late for them to flower now, mine flowered in spring, but also, I occasionally use a weak compost tea.
How are you overwintering them? Are you giving them a break? Sometimes plants NEED a dormancy period to stimulate flowering.