Veteran players will know what I'm talking about when I say things like, "rubies," "mantles," and "plates." There is an urban legend in Monster Hunter that states that the game knows when you are trying to obtain very rare materials and will do everything it can to make sure you don't get those materials. Of course, that is all false simply because the aspect of RNG is present.
From mid to late game, you *will* come across weapons and armor that you'll either have to craft or upgrade in order to obtain, and that they will require extremely rare monster parts. The drop rate of those items is about 1-2%, so... get ready and prepare to fight the same monster over and over again.
Those rare items can come in various forms such as carves, shiny drops, or as rewards at the end of the quest. It's very important that you don't feel discouraged if you aren't able to get the monster part at first. Keep going because eventually, you are guaranteed to obtain it one way or another! Not giving up and keeping your head up high is important, because the last thing you want to do is give up and have all that time go to complete waste.