Thanks Bob...great info.Keith
Please read https://www.rocketryforum.com/showthread.php?t=3456
I suggested that NAR S&T conduct additional static fires of the D9 and we observed that the AT D9 appears to be susceptible to propellant aging (oxidation) that causes ignition delays resulting in delayed peak thrust development of up to 0.6 seconds longer that shown in the manufacturers thrust curve. The AT D9 is a c-slot motor so IMO you should lightly sand the c-slot (not mentioned in the instruction sheet) to roughen up the propellant surface and remove surface oxidation to insure prompt ignition.
Old Estes D12 motors do not exhibit this phenomenon.