If this is of any help i have been discussing a similar taste loss issue in another thread. I've been through a process of elimination with different flavours & cleaning the tanks & dry burning coils etc.. I vaped some menthol & some other flavours, switched back to my original flavour & so on.
A few points to consider. If it's getting tasteless there are things we can do with our gear such as changing the coil for a lower resistance one. This has just worked for me quite well by switching from a 2.2ohm & a 2.5ohm down to a 1.8ohm . Probably other things we could do to.
What strikes me as something i never thought of before because it was too obvious is this. Your tongue has different areas & each of those areas detect different types of flavour such as spicy in one place, sweet in another, sharp, tangy, savory etc...
I'm not well read on the subject so i can't tell you the precise areas but it's enough to be aware of.... Stick with me here...
Using my own scenario as an example, i was vaping a nice coffee & a nice cappuccino (Jac Vapour & awesome) intermittently & they were very nice. After a few of days the taste was just not there, all i got was the misty dry neutral parp ! Looking at it now it seems so simple, i was stimulating one area of my tongue over & over with a samey kind of flavour & while it tasted good at first ,after those couple of days my taste buds just became numb to it. Too much of the same thing. I needed to let my buds recover by starving that area of flavour sense.
So i switched to some flavours that i didn't really fancy like red bull (Blah) & menthol (Yawn) & i vaped some cherry for almost two days (I like cherry) & then the coffee again just in the evening. After the 8 hour cherry sessions & a good feed my coffee took me back to heaven again.
I could go on but hopefully that explains my point. Any scientists please chip in !