Hi Ruth, I didn’t have time yet to look at and run the R code, but maybe the priors aren’t amenable to this calculation? One problem with Bayes factors is that the priors you want to use for estimation are often not the ones that make sense for Bayes factors:
I’m not sure if that’s the issue here but definitely worth thinking about.
Also, if the PPCs look bad then why not try to find a model that fits better and estimate the “effect” of interest with that model? That would be a better estimate to trust, regardless of what a Bayes factor comparing two other worse fitting models says.