Blocked sinuses as others have said, also dehydration will do it too. And you dont necessarily have to be exhibiting the classic dehydration signs either, just mild will greatly mute your taste vape wise. I think taste with vaping is effected way before anything else by mild dehydration. Your taste usually relies on foods, liquids in bigger amounts than is delivered by vaping and why you can still taste food and drinks no problem, but is why vape flavours seem to out of the blue for no reason vanish into thin air. Vapers tongue, I personally believe, is that. Not from being used to a flavour, but from mild unnoticeable in other ways, dehydration.
We all know vaping robs us in small ways of moisture. Try this. Take a big vape inhale and just an inch away from a fly screen blow slowly through it and see how much moisture alone catches on the wire. But more blows through than catches but its still pretty wet. Now multiply that by a hundred or more times a day, each day, each week and your losing quite a bit of body moisture through vaping. That loss has to be compensated for and if it isn't the first sign can be vapers tongue, mild deydration. I used to get what I thought was vapers tongue, flavour loss because I was too used to the flavour. But after I realised it was just mild unnoticeable in other ways dehydration, I have never had vapours tongue since.