Good thing that you removed your carbon cuz carbon will remove chemicals (including copper) from the water. Also, you have to add cupramine in the same proportion that you do your water changes. Your sand bed is probably absorbing some of the copper, but it shouldn't be absorbing so much that you're testing zero.
You might have a problem with your test kit. I had serious problems with my API copper kit when testing Seachem's Cupramine. I ended up getting Seachem's copper test kit. Make sure that the Seachem kit you get is the one with a small dip stick and white powder.
To keep ammonia down in your QT, put in a piece of LR from your display. If you have live bioballs or ceramic rings in your sump, you can throw that into a hang-on-back power filter.