Even the tallest mountains will be ground to dust with time. Given, of course, nothing sustains them (like tectonic activity which prevents mount Everest from eroding away too quickly, even making it taller each year).
Let time pass and the elements will erode away anything that isn't supplied materials quicker than it is being ground down.
And so the question arises: What could prevent floating islands from becoming floating clouds of dust or flat layers of floating dirt?
My best guess at the moment are wind currents taking matter (dust and particles) from the surface up to the top of the islands, so they settle as sedimentary layers over the island's surface.
This would mean the system grinding down the islands is the same keeping them from dissapearing or losing all geographical features. Which... I'm not too sure about.
In my world the islands are locked to certain points of a magic spherical layer that rotates over the planet's surface around its axis.
A good answer will provide a solution for the question that allows for (more or less) consistent renewal of lost material.
All suggestions are accepted and appreciated. Make those creative juices flow!