In each of my follow-ups, I mention that I am reaching them because I did not hear from them the previous time, and ask them to reply one way or the other.
It communicates to them that if they just reply a 'No', I would stop,
Also, I do not persist forever. My follow ups are always 4 levels deep, and in the final mail, I make sure I tell that it's the last one and I won't send another one.
So anyone like OP who's delaying it simply because they are busy tend to reply.
Again, I am not sure if these are ideal tactics, but they seem to work for me.