NONMEM Analysis for Future Bayesian Estimation

Note that even when CRCL is 5 (your lowest value) the renal comntribution to k is twice that of the "intercept": the contribution to total variability in k from 20% variability in intercept is, even for this most extreme case, (.2)(.01)/.025 = 8%. The contribution (of 1 SD of intercept) when CRCL = 100 is (.2)(.01)/.51 = .4%. It seems to me no wonder at all that you are unable to detect such a trivial component of variance. When NONMEM can't estimate a component of variance it drives it towards zero which is what happened to you. The proof is that the obj function is only trivially altered when this component is deleted. Bayesan adjustment with your smaller model should work just fine.