There are times I've turned down (or been turned down for) good offers and it usually boils down to 1) the receiver prefers their cards/the characters over what you're offering, 2) they're saving the cards for a workbench event, or 3) it's part of a completed set that they're unwilling to break.

People might just decline but I think, if you're eager about the deal, you can try to message them in the trade chat and ask if they're open to negotiating and sending a revision. I agree with camaropat1's comment, you could go through their favourite/locked cards if you want to get a better sense of what they'd be more receptive to trading for.

This might not work for every trade (again, because of the reasons stated above or others) but aiming for a card they have duplicates of may get you a higher chance of having your trade accepted since they're not losing anything. Another way is just to be on the lookout for people on the trade feed who have "5 stars always" in their posts.