Sarcococca humilis with yellow leaves

I have a sarcococca humilis in a large pot, and it has developed a lot of yellow leaves. It only receives a little dappled sun, and otherwise is in a shelted, shady location, in my south-facting courtyard. I thought that too much sun made the leaves yellow, so I've tried to give it as much shade as possible. My neighbour , however, has the same plant in a similar location, actually getting more sun than mine, and it has beautiful dark green leaves. I'm wondering if it doesn't like the pot. The soil is "The Answer", which I thought was nice and light and fast-draining. Should I just prune off the yellow (after flowering) and hope that the new leaves will be darker? Is there a special fertilizer I should be using?

 

Probably it has a nitrogen deficiency.

 

Thank you! I was just realizing as I posted my problem, that fertilizer was probably the answer. I get negligent with that!

 

Soilless potting media often quickly become badly leached.

 

thank you. I"m learning!