If you bothered to do any searching here on the site you'd see there are plenty of threads on this topic.
The only thing you're doing wrong is expecting instant results; in addition to the junk items and the multiplier items you also get boxes containing 10 and 30 donuts.
As mentioned it's 800 donuts spent (net) for every 100% gained. That means you're spending the donuts you won from the boxes also.
There isn't anymore advise to offer beyond, keep going. The mystery boxes will operate the same for you as everyone else, it works big time.
