I'm running a GMM specifying 2-4 classes on a continuous outcome for 4 time points. The 2 class model runs without a problem, but the 3 class model gives me a warning that the latent variable covariance matrix is not positive definite. Based on a preliminary LGC analysis including a quadratic slope, I specified the slope coefficients as fixed in the GMM. What is interesting is that when I run a 4 class model, it runs without a problem (i.e., normal convergence and no warnings). Do you have any suggestions for why this might happen for a 3 class model, but not a 4 class model? Are the fit indices (e.g., BIC) and the test for k-1 classes trustworthy when you get a warning like this?