Issues with Adding Custom Variables in Instantly Campaigns

Hi guys, I've got an issue with sending a campaign to Instantly via the built-in integration. I have a campaign in Instantly. I'm trying to add custom variables to it in addition to the ones already in the campaign and they don't appear in Instantly (although I don't see any errors in Clay). The only way it works is when I push all the custom variables as a single batch to a fresh campaign.