It has been suggested that putting in a trade route for an undemanded resource - this is not making sense!
Try an example - assume your city has (a) (b) (c) goods all bracketted as shown and that 'a' is being supplied to city X - it has been suggested that sending another 'a' caravan to city X will 'force' the release of a in your original city - as far as I know this is not a guaranteed strategy, but is the only thing that I have heard of that applies here
------------------
____________
Scouse Git[1]
"CARTAGO DELENDA EST" - Cato the Censor
"The Great Library must be built!"
"A short cut has to be challenging,
were it not so it would be 'the way'." - Paul Craven