This happened to my Gardenia, the leaves went yellow & the shrub looked really sparse. I tested the pH and it was too alkaline, about 7.5. I should have known as the Hydrangea nearby has pink flowers. I"ve dug in peat moss, watered with 1 cup vinegar in a bucket of water, watered in iron chelates & sulphur, a real blast of acidic material!. And it looks promising, lots of green shoots & buds forming & no new yellow leaves. I had previously given it a dose of Epsom salts but this didn"t seem to do anything. Also, even though we had a lot of rain in October the soil was quite dry underneath so the additional watering with Seasol may have helped.