I had this problem, all of sudden after asking to play Talksport I got a random Amazon music playlist (i use the built-in TuneIn skill), but sorted the problem by saying play Talksport from TuneIn, other channels weren't affected.
It will learn that is what you mean in time so you will be ble to omit the tunein.
Actually it's gone the other way, I used to just have to say Talksport and it worked ok, now I have to say Talksport from TuneIn to get it to work, although I should add this is only on my Echo Show, not a normal Echo.
I’ve had exactly the same problem Alexa just refused to play Talksport which is the only radio station l listen to and played ‘A station you may like’ instead. Problem solved by asking her to play Talksport by Tunein instead and works every time