It may seem like things go wrong just when you dont want them to, but thats only because you notice these things at these times. Take for example you lose your keys but dont notice them missing until the moment you need them, when infact they've been missing for a very long time.
This is definitely applying to me right now in MHFU. The other day I just started 4* ranked quests with the Elder and have been gathering in the Volcano looking for ruststones. So far, only one of those turned into a rust weapon (the lance). Still, I am having fun gathering lots of weapons to evolve (besides I need to keep going there for powder stone runs to finish the new upgrades to my farm).
There's a simple solution to this desire sensor and it is: don't care. If you don't care what you get and simply just say "I'll get it later" even after looking at materials needed you'll get it. I really don't care what I get and that gave me some rare stuff.
Example: The first 4 lagias I did I got a plate on each run then after finding out that I needed 5 plates for the Heavens Thunder longsword I started not getting it. Then one day I said "I don't even care anymore" and 2 runs later I got my 5th plate.
This also goes with the rath plates. Ever since MH1 I WANTED PLATES and my first plate was after 500+ hours of gameplay because I wanted them. Then Tri comes along and being experienced in the MH bullcrap I said "Man I'm never goin to get those plates so **** them"... in less than 100 hours I got 4 ian plates and 2 los plates (the 5 lagia plates included) when I was just trying to grind or help friends out.
Really all you need to do is not care and just play to play.