You mention bases, and for me, they are different than dioramas (a base is used as a 'backdrop' for 1-2figures that are not acting in any way) and it is difficult to say exactly what it is you do wrong.
First of all, where is it set? A North African desert is different to say the deserts of China.
Secondly, where is it more exactly? If it is set in a fairly settled area, the desert will be a little more beaten and tamed than if it's set in the middle of a wild desert. That means, the dirt will be packed and dirty. Oh, and EVERYTHING will be dusty, including plants and grass.
Thirdly, study pictures of the real thing.
http://www.popularmilitary.com/military-news/helmand1.jpg shows some marines in a desert-like environment. Study the ground: it is made up of very fine sand, 'dirtied' up by small rocks and pebbles and strands of dried grass.
The way I would do a base based on the picture above:
Mix plaster 50/50 with fine sand used for bird cages and perhaps a few grass strands for model rail road work and smear a thick coat on the base, making sure the coat is as smooth as it can get. If you want to use grass, place individual grass strands in the plaster when it is still wet and work he plaster up a little bit in each straw.
Make a very small batch of plaster mixed with tan paint - this will be your dust. Mix it until it is a very pale tan. Let this dry throughly and then grind it up with sandpaper.
Paint the base in a very light tan color - I recommend spray, but a light brush work could do as well and let it dry.
Spray the base with dullcote of your choice and sprinkle dust liberally over the base.
Finally, seal everything with another layer of dullcote if you think it would be necessary.