The problem here is the lack of warmth during summer, more than anything. Our coolness shortens and/or delays the growing season for both Pachypodiums and Adenium. Some years the smaller plants barely wake up. I've tried some that you have and they didn't do well. Our average daily high temperature from July through September is 72°, our annual high temp about 90°. We get near daily fog throughout early summer. It's an issue for the Pachypodiums.
I rely on a mix of 50% pumice which has served me well for a pretty good variety of caudiciforms. The Pachypodiums that do well here are lamerei, geayi, namaquanum, bispinosum, succulentum, sofiense, brevicaule (grafted), and that mystery plant I have been calling rosulatum. I keep my plants pretty root bound and they all get direct sun daily year round. Of the ones I grew from seed, there was a dramatic difference between lamerei and brevicaule in terms of the baby plants' tolerance for the soil drying out.
I suppose using more pumice in the mix could be a solution for the plants which have done poorly here, but I sense the problem lies elsewhere, and after that set of failures I decided to focus on plants which do better given the limitations.
