Hy George,
my first guess is that once you restrict your sample you loss the panel structure. I think that you have the --balance option in the smpl dialog. This allow you to mantain the panel structure. It work if your restrict option is for all the units. As an example, you have a panel of gdp for countries for different years. You can restrict your database for some years and that will mantain the panel structure.
In your case, I think the resctriction you are impossing destroy the panel structure of the database.
Regards,
Leandro