This is my first post so please bear with me.

I have a number of Zygo Cactii which live in a covered area outside. The roof is made of a clearish type of plastic sheet which gives 75% light. There are walls on 2&3/4 sides but the other side opens into a carport and the other little bit which is open is to the outside.

Some of the Zygo Cactus have leaves which are turning red.
One cactus in particular turned very red and then the leaves started to shrivel and the cactus eventually died. Another one is approaching this stage and if possible I would like to save it.

I have tried extra water. - Less water - moving to different spots - all to no avail.

Any help would be appreciated