Why do my pomegranate tree drop the developing fruit just after flowering,?

Question

its growing in a tub ,in full sunlight in Spain. It did have a dozen fruit on but only one is really starting to develope

Answer 1

The tub is probably your problem, restricting the roots and stressing the plant which normally grows into quite a sizeable tree. Plants subjected to stress will react by flowering in order to propogate itself. However, having flowered it has a limited amount of food to produce fruit due the size of the tub. It will do much better in the open ground.

Answer 2

Fruit drop is aggravated by practices favoring vegetative growth, such as over-fertilization and excess irrigation. Do not overwater it and do not fertilize it more than once per year.  Also pomegranates often drop their fruit between the ages of 3 and 5 years old. More established trees tend to keep their fruit.