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.

@lukas_catan I have the same issue. The 3:1 seems to work but 2:1 takes 4 resources to trade.

Also, if the AI refuses your trade, a note would be nice. That way I at least know I can blame the AI and not be unsure if the button even works.

@BobbyDaddy said:

I understand the AIs won't trade with me if I'm too close to winning, but when does that kick in? Does it adjust to the victory conditions of the game? I feel like when I have 8+ points I can no longer make trades, even if it's a 20-point game.

The AI won't trade if it feels it's competing with you for a spot, so that could even happen around 3-4 points. And I've seen the AI trade with someone on the verge of winning, even with the current behaviour.

This purportedly is all being reworked.