Arenaria The Pearl - not happy

Hilary1973

Hi

I'm new to this forum and am hoping for a little help please regarding a plant of mine that is not very happy at all. I have an Arenaria The Pearl which was very lush and appeared very healthy when bought and initially planted however it has slowly started dying off. The pot has good drainage and was planted in a mix of compost and sand (I planted a few other alpines at the same time and they are all fine), it gets lots of sun in a south facing garden.

Not sure if I have over or under watered? Or what the problem might be or if it is saveable. I would really appreciate any ideas anyone may have regarding this little plant. Many thanks.