Needs full sun and free draining soil, though in my experience, it also grows in half sun and heavier soil. When are you planting them, spring or autumn, and in what situation in terms of light?

It sounds like they have not established and/or there is a problem with the roots. Dig up and have a look at the root system.

At the moment I have 6 aubretia growing slowly from seed in 3" pots, using multi-purpose compost with some sharp sand included in the mix. They like good drainage. As long as they are in the sun they should grow well. Just one other thing. When you are transferring them from pot to the growing area I would tease out some roots from the soil ball and water sparingly for the first few weeks if it does not rain to encourage them to establish.