Hey there, hearing as how you have mechanical switches, I think the issue lies in the switch itself. You could:
1. Tap Hard on the key (like smashing it repeatedly for about 10 times)
2. Remove all the keycaps, take out the screws, and desolder the switch (if you know how or have anyone with experience on keyboard switch desoldering) after that you may A. take apart the switch and clean the parts inside. Or B. Buy new switches and solder them to the board. Desoldering is needed because your keyboard isn't hotswappable (thats not necessarily a bad thing)
I had the same issue before and did 1. a lot then just did 2. Because it returned sometimes