Answer: Agapanthus are usually left in the same container for a number of years as they seem to do well enough when fairly potbound so I would not recommend disturbing the roots unless it is now in a severely undersized container. (If you repot, August is the time to do so. Wear gloves as many people are sensitive to this plants.)
Since you had good results before, I would tend to look to the weather (perhaps it was extra cold last fall before you brought it in, cloudy late spring, then dry summer) and possibly a lack of nutrients due to the soil being older and somewhat depleted by now, plus the fact that the plant is now larger in relation to the size of the container.
Make sure you are watering it enough during the growing season -- it needs ample water when in active growth, and make sure you are fertilizing weekly with a water soluble fertilizer with an analysis of 10-10-10 plus minors or similar proportions. You might also try an occasional dose of kelp based fertilizer (supplies many micronutrients) and/or compost tea.
Good luck with your Agapanthus!
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