Why is my Datura Metel Ballerina losing all of it's bottom leaves?

are the leaves a yellow pale colour? if so it could be an infestation of two spotted mite aka Red spider mite. They are tiny little creatures normally visible with a magnifying glass. They feed on the underside of leaves.

They tend to favour hot dry atmospheres e.g greenhouses/conservatories, so if possible, spray it with water and keep air damp, but not to damp or this can cause mildew. There are sprays you can buy. Give it a feed to to give it a boost.

No signs of mites. And the leaves are not yellow or pale.
They remain green and then just drop off. I'll try to remember to take a photo of it and post it here.