Why can't I scout this city?

As city pop decreases, it is harder for the attacker diplos to succeed.  Strangers stand out more in smaller cities than larger cities - is how the description puts it, approx.

I will send more then, as many as I have.

I thought about that, and have sent more, only 8, but guess I'll know when they get there. 

Guess I should give more info... I scouted the city before my army arrived, and got a report back.  I've been thieving it since I found it, and can still get goods from it, but scouts and spies fail as long as my siege army is there. And, this has happened before, when my alt sieged a city. I just accepted it as part of the game. The more I think about it though, the more it doesn't make sense.

I can send them, but they always fail, and don't return.