Change of NSIG or R matrix

Dear all, I have a naive question regarding the modeling building process in NONMEM. With more and more covariates added in the model, I often come across an error message saying that "ERROR 134", or R MATRIX SINGULAR. After searching from the internet, I learned that changing NSIG in $ESTIMATION and MATRIX=S in $COV would be helpful for both problems respectively. And from my own experience, it dose help with the modeling building. However, my concern is, I used different NSIG and MATRIX in the previous steps. Is it proper to use different NSIGs and MATRICE in a single model building? If not, could you please explain this a little bit? Thank you in advance! Best Regards -- Xinting Wang