Glad to see you working on it again. Sampling failures are something I have been struggling to intuitively grasp over time, but I have helped build it over time by searching over many threads in these forums using some filters.
One reason sampling fails that I do grasp intuitively, is when a parameter is being sampled over an impossible region for it, like when sampling a sigma over negative values.
Other reasons for divergences/failures, are a bit more complicated to understand, and I realize now I should be working on that understanding myself.