nm7 IMP method question

Dear users, I recently started using IMP methods offered by nm7. In the first IMP step, I had 100 iterations and the OBJ was quite stable already around 20 iterations. In the second IMP step to obtain SE of the estimate, as "$EST METHOD=IMP EONLY=1 INTERACTION NOABORT NITER=20 SIGL=4 NSIG=1 PRINT=1 ISAMPLE=300 DF=4 IACCEPT=1", the OBJ value kept increasing as iteration moved. at the end, the SE for the model parameter estimate was not bad (<50%RES) could someone help me understand why OBJ continue increasing in the second IMP step? thanks.