Problem with estimating sigma when using M3 method

Hello I'm fairly new to nonmem, I'm currently trying to model a phase 1 study with BLQ values, while the run was successful with no error message, my residual error has a %rse >70 and a confidence interval that includes zero. Here's my code: $ERROR LOQ=0.1 IPRED = F SD = THETA(8)*IPRED DEL = 0 IF(SD.EQ.0) DEL = 1 IRES = DV - IPRED IWRES = IRES / (SD + DEL) DUM = (LOQ -IPRED) / (SD + DEL) CUMD = PHI(DUM) + DEL IF(BLQ.EQ.0) THEN F_FLAG=0 Y= IPRED +SD*ERR(1) ELSE F_FLAG=1 Y=CUMD MDVRES = 1 ENDIF $EST METHOD=1 INTERACTION LAPLACIAN PRINT=5 MAX=9999 SIG=3 SLOW NUMERICAL MSFO=*.msf $SIGMA 0.38 ;[P] sigma(1,1) (estimated in a previous model) Furthermore, when trying to fit this model to my phase 2 dataset, covariance step fails when I implement it. Any suggestions are welcome Thank you