Answer: Your growing conditions sound adequate, however you might want to try the following: promptly remove and and throw away any affected leaves so they do not serve as a source of reinfection, open the shades to allow more light to enter, fertilize according to package instructions with a water soluble fertilizer for foliage plants, water moderately so the plant is neither soaking wet nor bone dry (check with your finger and see if the soil is moist or not), and rinse the foliage top and bottom sides once a week in tepid water -- a kitchen sink sprayer is ideal for this. Based on your description I am not sure what the problem is, however it is possible that an insect such as spider mites is causing the problem. If so, washing weekly with plain water (and/or several applications of insecticidal soap according to the label instructions) should help as would increasing the humdity level around the plant by placing it on a tray of pebbles with water nearly to the top of the pebbles but not touching the bottom of the pot. I hope this helps.
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