xtmixed covariances: why it doesn't work??

#1
Hi,

I'm trying for weeks to run the following model:
mi estimate, variance : xtmixed MathG5 MathG4 , || classID:, covariance(unstructured) || studentID: , covariance(unstructured)

This is only the first model, I want to add a lot more covariates and interaction effects, but if the first model already runs for three days and still doesn't give any results, I guess the other models will need a year!
The multiple imputation is not the reason, because I also tried to run the model
xtmixed MathG5 MathG4 , || classID:, covariance(unstructured) || studentID: , covariance(unstructured)
on the original data without imputations, and that also doesn't work .
I also don't think that there is something wrong with the data, because this model does run:
xtmixed MathG5 MathG4 , || classID:, || studentID: ,

Anyone countering the same problems and ready to give me some advice? I'm desperate! I tried everything I can think of, I augmented all kind of memory options that are augmentable in Stata IC 12.

Thanks in advance and happy new year!