I've spent a few days trying to google this mystery, but can find nothing. Not only coke, but also similar drinks like 7 up. I can taste sweetness in fruit juice, chocolates, and desserts, but never in most pop drinks.
I was reminded of this because I tried drinking a can of coke recently, and it just tasted like sparkling water. Same with 7 up and some brands of fizzy orange / fruit pops.
Fanta I can taste fine, but that uses a mix of sugar, aspartame and acesulfame which is probably why.
I think its because the ones I cant taste use HFCS maybe?
First, there's a difference in Coke from different countries. I remember the first time I ever had a can of Coke on a trip to the States (a friend and I spent Labor Day weekend on a trip to the PBS station in Spokane when Sylvester McCoy was on a cross-country tour of PBS stations to promote Doctor Who). Saturday night was a Coke-and-pizza-and-Doctor-Who night at the motel (the station manager had introduced us to four guys at the station who'd also made the trip: "You gals're from Calgary - there's four fellas here from Edmonton, maybe y'all know each other"), and that was my first time drinking American Coke. My friend tasted it first and she got a funny look on her face: "This doesn't taste right."
So I tried some and had to agree - the taste was
weird. I assured my friend that the cans weren't damaged, so there was nothing wrong with the Coke itself, and the guys from Edmonton had been drinking it and were still alive... so maybe there were different ingredients. Naturally we didn't have any Canadian Coke with us for comparison, so I didn't get to compare any until a couple of days later. I'd brought a few cans of American Coke back with me (odd taste, but not unpleasant once one got used to it), and when we got to a gas station in Alberta I bought a couple of cans there and had a look at the ingredients list for both - and yeah, they were different.
So maybe you might like the Coke from a different country.
Or maybe you might prefer more citrusy flavors - right now I'm working on a bottle of mango-flavored pop (it's a store brand only available from the place where I order my groceries).
Possibly normal Coke won't work, and a different variety might? We used to have Vanilla Coke here and it was wonderful stuff. But not enough people bought it so the stores don't carry it anymore. Same with Cherry Coke - that's a rare treat, though I can sometimes get Cherry Dr Pepper.
And then there was the time when I found a delectable combination of cherry-chocolate flavored cola - not one of the brands I recognized, and it was only available for a short time at one convenience store in the neighborhood where I used to live.
Or it's entirely possible that you just don't have the kind of taste buds that recognizes any of this stuff. In that case, you're healthier for it, because you'll be far less likely to develop an addiction for it.
Check the labels. 
