How tf are you supposed to set up trade agreements with your neighbors/vassals?

So I have a vassal I defeated in a war years ago in-game time, I worked hard on diplomacy with them so they have +500 towards me, we have positive relations, and they’re allowed diplomatic independence and stuff, we have a shared research and trade agreement.

However whenever I go and trying doing a trade negotiation these mfers always refuse. Like, I’m talking I’ve set up to give them shit tons of consumer goods and alloys, part of my fleet, and entire star systems, and these bastards hit -1000 when I ask for a single monthly mineral in return. They’re “hegemonic imperialists”, so these type of people just not trade? If they’re economically worthless to me I will invade them again and then purge their people.