Agapanthus

I have had an agapanthus in a pot for 5 years and talk about touchy! The first year I repotted it - bad move - no flowers. The second and third years - loads of flowers. Last year it looked really in need of repotting again - the roots are all over the top of the pot - one flower. I don't want to put it in the ground but I think I shall have to put it in a much larger pot. Probably won't get any flowers next year!! As for the leaves looking yellow I know they like free draining soil and I haven't fed mine with tomato feed just the occasional watering with a general purpose fertilizer.

They like to be divided as well, I did this with mine this year and i have a flower on one of the divisions after planting it in the ground

Agapanthus like to be pot bound in order to give a better performance. But too pot bound and they wont flower! A difficult customer!

If its an evergreen one, then it may be winter damage form the cold and frost esp this winter!
It could be too much water or lack of drainage, perhaps cut back on the watering and see if the plant picks up and green up.
Or it maybe a fungus (not the word im looking for but some sort of disease). best to cut off infected leaves and destroy, and sterilise any tools used. The rain or water splashes will spread the spores onto other leaves