I think you're possibly right about the dryness Len, as some have barely grown at all this year, possibly due to that but that said, whenever i dig the gravel up it's always wet beneath only a few inches down a few days after watering so it doesn't seem to drain out straight away. Despite some not particularly growing well this year though, others have so it certainly doesn't seem a major issue, I was going to see how the troublesome ones do next year when they haven't got the "just settling in" excuse
The P. Pringlei at the back in the second pic, both C. Strausii, the small T. Bridgesii and both M. Geometrizans as well as the two long-spined T. Pachanoi have put on good growth this year. The ones that haven't are the short spined T. Pachanoi, the two T. Chilensis, the two T. Spachianus. Most the others haven't been in there long enough to judge just yet but are looking okay so far, certainly the tallest one there, the Stenocereus (can't remember off by heart what it's full type is) is putting on some fresh growth even now.
