I've had this Dieffenbachia for about 8 years. It's 5 feet tall and is fairly healthy, but it is having issues.

Symptoms: Clear, sticky substance on stems and stock, but there is no substance on leaves (see photo). Flower buds turn brown and die before opening.

One to two leaves turn yellow/die per week but, otherwise, the leaves, including the underside, are clean and healthy. The plant has not been moved and gets good, natural light. No evidence of scale (I had a plant once that had this and can recognize it).

I can see no bugs anywhere. I'm not a knowledgeable gardener by any stretch, but I do know honeydew is a sign of aphids. However, wouldn't I at least see some specks (bugs) somewhere?

Any help/advice you can provide would be very appreciated.