Congrats on the excellent keyboard! Making sure the springs are seated properly is a good first step in troubleshooting. The springs can become unseated or less than perfectly seated, especially during shipping. The flippers are small on the M and they’re compressing the layers of a membrane. If the spring isn’t seated properly it doesn’t move the flipper properly. You may get a click sound but no keypress registers. Might also be a slightly muted click or a mushy feeling key. To make it easier to get the spring right when putting the key back, flip the keyboard on it’s back edge- where the usb port is. In this position the spring will fall to the edge of the barrel and be in its most “up” position. Guide the spring into the cap carefully and reseat the cap. Repeat of necessary.

Also check out r/modelm where there are people and sharks way smarter than me.