I'm one hundred percent sure that if the Sorceress doesn't accept your payment for the secret... it means you're offering her resources and Plague Zombies :) and she asks for the most ordinary ones!!! Without any upgrades:)
I don't know, maybe everyone already knows this, but just in case: you'll definitely have to visit the Sorceress twice - the first time, even if you bring everything, she'll still send you away. For appearances' sake, you need to return to the mine with the magical ore (even if you've already conquered it), and then visit the Sorceress a second time. Only then will you have the opportunity to upgrade your troops.
