Water logged soil and nutrient deficiency came to mind when I read that it is going yellow. Check the soil with your finger - is it damp/soggy or dry to the touch? If it's damp, the plant has been over-watered.
As for nutrient deficiency, this results from potting mixture that is not acid enough.
Either way, I think a re-potting of your plant and a freshening-up of the soil is in order. Use potting mix that contains some peat. When watering, add some iron sulphate to the water. Or, you can encourage healthy foliage by sprinkling some nitrogen rich fertilizer granules around the root zones and watering them in immediately. Also make sure you feed it potassium rich fertilizer to encourage those flowers to bloom.
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