This is not necessarily a bug. Sometimes players will decide they don't want to trade with you specifically. This can be because you are ahead on points, competing for a resource intersection, are about to get longest road, etc.
AI opponents will very rarely accept a trade with a real player.
As far as I understand, this isn't a bug. It just isnt a quantified trade. I thought it was a bug too at first! But it's just the opponent telling you "I want some wheat, what would you like for it?"
You can select the ressources you want for it and return your answer to see if he accepts.
You can also do the same and ask for one ressource without asking for anything, and they will tell you what they want for it... Sometimes!! AIs are tough when the trade comes from you.