I'm wondering how long its been in the same pot, for one thing. Received wisdom regarding Agapanthus is that they like to have their roots cramped - personally, I've not found that to be true at all. When I kept them in a pot, I got one flower and then none at all - planted out in the ground, flowers every year and gets bigger every year. I'm not suggesting you do that with yours because this one's frost sensitive, but if its been a while since you repotted, it might be time to do that -either a larger pot, or split them and pot into 2 pots.
The other thing is feed - what have you been feeding with, and when and how often?

I agree Downsizer,there is a limit as to how cramped the pot can be..so a good suggestion to split it..but still overwinter them as usual..The first time I split mine,I didn't get many flowers that first year,but have ever since..

In a pot, they will need regular feeding also. I would use a feed with high nirogen, low phosphate, and moderate potash, too--21-7-14, 15-5-10, and suchlike are good. Also make sure that they are getting enough direct sun when they are actively growing.