No problem with other cards for me. Only with Duelist the salvage option doesn't appear.

I've found several that aren't salvageable. Will provide more info later on which specifically; but, I can't find any rhyme or reason to what decides if something's salvageable or not. Rarity seems to be a factor, but not the only, as some common can't be, but all (I think) epic can't be.

It's just an issue mate. Every rarity has its own value for salvage. No point to maintain any card (also increasing its chance to appear in game if you don't want it).

If you have a list it's great because the devs will fix them easily. I have few unlocked cards so I don't know if there are others or how many they are.

This problem comes and goes since patch 1.0.3.7. I've had it happen on several cards. Try Again later in the day or tomorrow and it will probably be fixed. As a side note epic cards can definitely be salvaged.

Question, are you online? I made the mistake of try to salvage while offline which doesn't give the option.

Yep mate, the cards on the side of Duelist in the image had the salvage options. For that reason I found it very peculiar.