I planted 6 calla lillies bulbs in 2 pots a year ago and had a profusion of flowers. I over-wintered the bulbs in the same pot and this year I had 1 flower instead of 50 but lovely foliage. What happened?
Like all flowering plants, Calla lilies, which are in the plant family Zantedeschia, require the right conditions in order to produce flowers. These plants originate from tropical marshes and grow from Rhizomes (modified plant stems that resemble bulbs or tubers). They are not true lilies. You say that you over-wintered them in their pots but have not indicated if the foliage was allowed to die back or where they were stored. Insufficient sunlight is often the cause of bloom failure and, depending on the size of the pots, overcrowding may also be an issue.
Growing requirements are consistently moist but well-drained soil; a full sun location; and a monthly application of a balanced liquid fertilizer until the flowers have faded. Calla lily plants then require a dormant period. Spent flower stems should be removed, withhold water and allow the foliage to die back but do not remove the foliage until it has yellowed and dried. Remove the rhizomes from the soil, allow them to dry and store them in a paper bag or wrapped in newspaper in a cool (10-15°C), dark location. Check them periodically to make sure that they are not too moist or too dry.
The rhizomes can be repotted in late winter and the plants moved outdoors in the spring. They should not be placed outdoors until after all danger of frost has passed.
I’m including a link to a previous Toronto Master Gardener answer with further details about growing and storing Calla lilies:
Here are some links to some additional information: