lagged absorption and empirical input function

Hi, I'm trying to implement an absorption lag-time into a model described by differential equations and using an empirical input function. My initial idea was to define a MTIME(1) corresponding to the lag-time and to use MPAST(1) to set the start of the input as below. DADT(1)=(DOSE/V)*INPUT*MPAST(1)-K*A(1) However, I only end up with no input at all in the system resulting in simulated DV of 0 ± residual error. The issue seems related to the use MPAST with the input function but I don't know how. Would anyone be kind enough to point me in the right direction? Thanks, Pierre-Olivier