Being taken away from Mum and siblings is a terrible wrench, and they need time to adjust. Its good that she is eating anything and using her tray. 6 weeks seems very young, but then I breed pedigree cats and those leave home around 14 weeks. Even at that age, it can take some of them a week to settle.

I would give her anything she likes just to keep her eating. Then try taking whatever is her favourite, and mix in a little of what you want her to eat. Then over time gradually increase the amount. Tuna is fine as the odd meal, but its not good as a permanent food anyway. Have you tried tempting her with anything else - cooked chicken? That said don't try her with too many new foods too quickly as that could upset her tum.

She sounds like a typical girl They love to test you and make sure you are the right owners for them.