New Guinea Impatiens new, potted, one leaves dying from lower tier upward

Hello,
I'm new to gardening. About 1 month ago I potted 3 one quart NGIs in well drained ceramic pots about twice the size of their store containers. In southern California they sit on an outdoor shelf which is shaded from the sun by a solid roof. Two are doing beautifully but one has lowest tier of leaves yellowing, withering, and dropping off - about one or two per day. That plant shows no new shoots at the soil line, while the other two do. All plants are in flower with new buds. Moisture meter shows each of the pots to be the about same, at 3/4 of the scale, and I water (with tap water) about every 4th day, if the indicator drops below half. The ph of the ailing plant is slightly more acidic (less than 1/8th of the scale) than the two healthy plants (which are near the bottom of the scale)
Searching the web I find "yellow leaf spot," but that does not match what I see, and nothing else pertinent about the plant with "yellow leaf" of just plain "dying."
Any ideas on what to look for or any similar experiences shared would be appreciated.
Bill
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Bill's News wrote:

This may be my answer, some time will tell. Impatiens are acid loving plants - I've now learned. So I've fed them with the Azalea food and will monitor. The yellowing leaves do not exhibit the cited characteristic of green veins but the Ph was very low, although slightly more acid in the problematic pot?
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