|Two years ago I purchased a Monrovia plumbago plant from Atlantic Gardens (the only local retailer of your plants). The plant was in a terrible state at the time- nearly dead, and with a gritty substance under the leaves. The store explained that with the extreme winter temperatures that year many of the plants had suffered, that the grittiness was merely the residue of a pesticide spray and that the plant would quickly return to health given good conditions. I believed this, as I was very familiar with plumbago from my time spent in California and in Australia, and knew it to be a highly pest-resistant plant. And since I had been searching for a plumbago plant here in Canada for some time, I bought it anyway.
But unfortunately it did have some pest- a pest resistant to all the usual household plant sprays, and a pest which infected the
|Plumbago is usually trouble free so there is no "usual suspect" you are right. Based on your description I think you might be seeing evidence of a cottony scale insect problem which eventually does cause yellowing, the stickiness could be caused by the sucking and the black spots could be mildew -- and there would be no webs as with spider mite. Without seeing the plant however it is hard to say for certain. Here is a photo, note the white dots ...
Scale can be treated in the crawler stage with insecticidal soap or can be sprayed with horticultural oil to smother it. I'm sorry you are having trouble with your plant.