I had already asked a similar question here, but I'm experiencing the same problem for a different data-set and for a different family of mixed models.
My response variable is a binary outcome of whether an individual hunted a prey or not (0 and 1). I'm trying to see how this hunting behaviour change with time (across 31 trials). My data-set consists of 242 individuals (ID) nested within 12 different colonies. Both colony and ID were converted into factors, while the fixed effect predictor (trials) is an integer ranging from 1 to 31.
Since the model using maximum likelihood (binomial glmer) resulted in a singular fit, I used a bayesian model (from rstanarm).
(I used only 2 chains because running the model with the default 4 chains resulted in issues with the RAM while visualizing the plots).
Model diagnostics were checked and the model seemed to be a reasonably good fit (no divergent transitions, rhat < 1.05, very low autocorrelation).
From this model, I generated random slopes plot using sjPlot package as follows:
And I got the following plot
As you can see, some lines are curved. I'm wondering how regression lines cannot be straight. I'm not sure what went wrong with the model. Any help will be much appreciated. Thanks!