Professional teeth whitening (Boutique Whitening) didn't whiten my teeth - why not? What can I do next?

I used 10% carbamide peroxide overnight for 1.5 weeks, then moved onto 16% carbamide peroxide overnight for 2 weeks, then 6% hydrogen peroxide for 1.5 hours a day for 1.5 weeks. My teeth haven't seemed to whiten at all. I'm keeping to a diet of light foods only, no coffee + tea. I'm using my Invisalign trays to bleach my teeth, as the dentist directed. What should I do next? I've spent £425 with no results and I'm 25 so I don't think my teeth are too badly stained. I'd like to get to shade BL2.