OMEGA selection

Dear all, I am fitting a PD response, and the equation goes like this: total response = baseline+f(placebo response) +f(drug response) first, I tried full omega block, and model was able to converge, but $COV stop failed. To me, this indicates that too many parameters in the model. The structure model is rather simple one, so I think probably too many Etas. I wonder is there a good principle of Eta reduction that I could implement here. Any good reference?