Hey Bill,
If you are referring to registering the players to play in a particular competition, then you could be outside the registration window. Players not registered by that date can;t be registered until the next window.
Not sure what you mean by 'fill down to S7' but it sounds like you are on the Tactics screen, so you are picking a team of 11 plus 7 subs (S7?). The names below the S7 slot are still in your squad, are probably registered just not picked in your match squad of 11+7. Click and drag a name from below the S7 slot and drop it in the slot, the two players should swap in the list.
Any unregistred players for the competition will have a small icon by their name with 'Unr' which means they can;t play in that competition.
Hope that makes sense