I once contacted them directly through mail (and not via the "Support" button) about an offer that had disappeared from the store, thus not showing up in my pending list. I didn't expect them to do anything about it, because it was a limited number offer and I completed it way after it was gone.
The team was very quick (and really polite, too!) to reply and actually granted me the plat. You just need screenshots as proof and your Fyber ID.
Fyber is my favorite provider.

Tapjoy on the other hand doesn't give me the reward of the nicer offers at times, and the two times I contacted their support team, they told me they already granted me the plat (no trace of it in my purchase history) the first, and didn't even bother to reply despite my screenshots the second. I waited a month (July 9th to August 20th) before sending a second mail. I am to this day still waiting for their reply.