Usually I will analysis the cases where it does not work:
- lots of divergence. solution: more informative prior
- some chain is a flat line. solution: likely initial start point problem, manually supply
start - bad mixing (random walk). solution: this is a tricky one… likely the mass matrix is adapted to a local geometric, manually start point might help. otherwise, you might want to try smarter mass matrix adaptation like the one in https://github.com/dfm/exoplanet