Plates and Rubies are rare drops.
You can get plates from any rank Rathalos, but the rubies only come from High Rank Rathalos and higher.
One way to farm rubies "quickly", is to cut the tail (Focus on it, and you can do it within a couple of minutes of starting the quest). If you do not get the ruby, abandon quest and start again. Other than that, you would have to just keep killing Rathalos until the rare drop shows up on your quest rewards.