You do lose some DXM in an Agent Lemon extraction simply because the whole ordeal is so imprecise. You can only be so accurate with zip-lock bags after all. Also, Robitussin DM isn't a good choice. It contains more active chemicals than Robitussin Max Cough and even contains LESS DXM. You probably got a lot more contaminants and less DXM in your version of Agent Lemon than someone with the proper syrup would have. Also consider DXM concentration is NOT EQUAL throughout the Agent Lemon solution no matter how pure it is. You can't simply weight it out and split it between two people!

DXM would never take more than 8 hours to kick in. 2 is good and 3 is a bit tardy in my book. A better source will yield a better result.