While the New England data doesn't show that mask mandates are working, it has been lacking a control group since the New Hampshire mask mandate.
On the other hand, we do have an active experiment in the United States with the Dakotas.
Both saw huge spikes, and both saw huge drops, regardless of mask mandate:
The obvious answer is probably the correct one. Because people who are conscientious about not spreading the disease and believe the science do wear masks regardless of a mandate. And those who refuse to wear masks for whatever reason, continue to do so regardless of a mandate. I've certainly seen plenty of that around here.
The problem of effectiveness isn't mask mandates per se. The problem is mask mandates without teeth. AFAIK, there are no fines specified in the Governor's executive order. I have yet to see a single worker challenge a maskless person, even when there's signs on the door "do NOT come in without a mask." The Governor has said before he doesn't want to hear from the public "ratting" out businesses who don't comply, and I'm now hearing the Sunapee Ski Area is a joke of compliance, with crowds doing no social distancing and people not wearing masks. It's like saying we're putting in drunk driving laws to protect the public, but there will be no enforcement or fines. Or repeal them altogether because I have a constitutional right to freedom, and that means I shouldn't have to be sober to drive. Yeah. Let's see how that works out.
The fact we even have to mandate common sense is ridiculous, but necessary given the sad behavior of too many.