High concentration of ferric ion catalyst decreases rate of decomposition of hydrogen peroxide in the Fenton reaction
In this paper, a similar reduction in rate is observed at concentrations of H2O2 above 0.03 M final H2O2 concentration (which is about 0.1 wt %). They attribute it to $\ce{HO.}$ reacting with H2O2 (to yield $\ce{HOO.}$) more quickly than it reacts with MB. The result is that $\ce{HOO.}$ is much more abundant than $\ce{HO.}$ and its lower reactivity means a slower rate of reaction with MB.

Based on their data, I would recommend repeating the experiment with much lower H2O2 concentrations.