Do you get a lot of mobile traffic? There have been some big changes in AdWords with regards to mobile in the past couple weeks; I can't right off the bat make a connection to your problem, but ... it's a thing.
On a very high volume traffic site I had been averaging only like 3% unmatched when I was only running Google as an ad network. This morning I added almost all the ad networks and my unmatched % is now close to 10%. I should have listened to everything I already knew about Google and that is whatever a Google employee suggests you do - do the opposite.