In my current Civilization game, I have 2 x silk and 1 x marble. The tiles have improvements built on them and they show up in the resources info panel under "Luxury resources". However when I go to the diplomatic screen with another civilization, it shows as if I had nothing to offer. What's up with that? Is there a way to see how else they are being "consumed"?
Odds are, the resource issue isn't on your side, but on theirs.
If the civilization you are trading with already has a luxury resource, it won't show as being available to trade to them. That's mostly to protect the AI from being abused by humans, who if they were allowed to trade luxuries the AI already has to it would probably find a way to take advantage of it - so they simply don't offer you the possibility.