Great cashback, eventually. Persevere.
Booked three rooms November 2015, for a stay in January 2016. Cashback didn't track so I raised a claim. This was originally declined with no reason given.
At that point I wrote to Quidco and explained that I'd definitely not used another referring agent / discount code, and that Marriott had given no reason for declining my claim.
Quidco reopened the claim and it was paid, end of June 2016.
I'm happy with the outcome, but we shouldn't have to jump through such hoops. I say we as, from reading other's reviews, I'm far from the only person to have had claims spuriously denied by Marriott.
My claim as for £30, so not insignificant. My advice, if you find yourself in the same situation, is persevere and follow up with Quidco.