i have several perennial hibisus, some seed pods are developing normally, while others get to a size just a bit smaller than a marble, then turn yellow, stop developing, then turn brown and just die. other pods on the same plants are devoping just fine.they are all big healthy plants with no disease or insect that i can find. does anyone know why this happens? if it matters, i have:

red luna

old yella

lady baltimore

turn of the century

same thing on all plants...thanks, donna