There are several possible reasons for the plant not to bloom, one being lack of nutrients another lack of water and yet another the possibility that the freezing temperatures have weakened the plant.
When growing agapanthus as a container plant, you need to fertilize it regularly with a water soluble fertilizer according to the label instructions. Do this when it is actively growing in spring and summer. You could also apply a top dressing of compost to ensure a good supply of micronutrients.
The plant likes an evenly moist soil during the growing season, so keep it very well watered during spring and especially summer. This will likely mean daily watering during the hottest weather.
The plant is said to bloom best when slightly potbound, but if yours is very crowded you could divide it or repot it in the spring. An overly crowded plant will be difficult to keep amply watered. This would also allow you to refresh some of the potting mix.
Since you are bringing it indoors for the winter, I would suggest bringing it inside whenever temperatures drop below about fifty degrees, this will avoid shocking the plant with cold. Inddors keep it in a very bright location and water enough to keep the soil barely moist.
In the spring, remember to take it outside gradually increasing its exposure to the sun over a period of weeks to allow it time to adjust to the brighter light.
Good luck with your agapanthus!
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