Buxus plant

Hi

I am a total novice and have recently purchased 2 buxus plants.
They are still in the pots and they are watered regularly if we have no rain, they are in a shaded area but the sun light does come around in the afternoon.

The leaves have started to go yellow, I have added miraclegro and some feed fertiliser.
Is there anything else I can do salvage it?

Thanks for your help!

Welcome to the site 

I'd also inspect them closely to see if there are any signs of the box tree caterpillar, and do an internet search for images of box blight.  It might be neither of these things, but best to check it out.