IIV decrease in mixture model

Dear NMusers, I have a PD model describing viral infection process. There is variable Vo for initial viral load and it has IIV, omega(V0). When I fit the data with simple model, omega(V0) is 52% and covariance step is ok. But when I try mixture model, omega(V0) becomes very low, and covariance step is not finished. But if I look at post-hos values of Vo, there are two peaks, for 2 groups of patients. Do you have any idea why it is like this? I mean it seems strange that omega(V0) decreases so much. Thank you all in advance, Svetlana