I don't know what's different with what you buy compared to what I buy, but I've spent a small fortune in the past year, and I've gotten prices down to the bare minimum - but I've not been able to make savings with cashback sites.
I certainly look around and I acknowledge that on many things there isn't much to be had, but various clothes sites I use give me back a little bit of cash and for the sake of a click it's no big deal. My mobile phone deal was ridiculously good, but as you say, they arn't always. Virign media are currently offering £142 back on any new cable connection and I'm moving into a house next month so have taken advantage of that.