As the members of the Clan complete challenges and collect points for the Clan, new reward tiers become unlocked for the entire Clan.
Rewards selected from Clan Games are added to your Treasury.
Everyone sees the same rewards for Clan Games, except for players who are lower than Town Hall 7, who see Gold offered in place of Dark Elixir rewards.
The quantity of Elixir, Gold and Dark Elixir rewards is adjusted based on your Treasury capacity. Treasury capacity is affected by your Town Hall level and the level of the Clan Perk affecting your Treasury capacity. This is why your reward amounts might differ to another player's rewards.
Everyone sees the same Magic Items as rewards, and everyone is offered the same quantity of these Magic Items.
If your rewards disappeared, below are some potential reasons: