I apologize if this question has been asked previously.
I conduct mixed-effects model analyses in R and MATLAB, so this question is more conceptual than technical. MATLAB and R both produce p-values for each coefficient for GLMERS/fitglme.
In some cases, I may have a model in which there is a 3-way interaction in my fixed-effects structure with just a random intercept term. Here, most of my fixed effects are significant (p < .05).
However, when I add random slopes to the random effects structure, the significance of the fixed effects diminish. I'm assuming this has to do with the random effects accounting for individual variation, but I was curious if there was a fuller answer.