Hi, Mary Ann. Worn-out soil is a possibility because I've been gardening intensively for 30 years in my present garden without doing much amending. I'm gardening in good sandy loam with lots of deciduous trees on the property, so I may have depended too much on the soil's ability to stay good with only the addition of the leaves from those trees. This year I did have several yards of compost delivered to my house and I spread it out fairly evenly in all parts of the garden, so maybe that will make a difference.
It's likely to be due to more than worn-out soil, however, because other plants in the same beds are performing as well as ever.