I was fighting a Yian Kut-Ku (I think that's the name), because pride would not allow me to just submit a scale that I found on the ground and call it a day.
Boy, was I not ready. My weapon seems to bounce off of it half the time, even at full Sharpness, and I have no counter to the fact that it shoots fireballs.
My Palico was pinned down, bravely buying time for me to chug a Mega Potion before I returned to the attack. The Kut-Ku, apparently sensing my desperation, turned and casually spat a fireball. I was stunned and the last of my health burned away while I was helpless.
MH4U. I'm gonna have to get Generations next paycheck, because I want to do this multiplayer.
Also, do monsters have spots that are more heavily armored? On certain large monsters, attacking them from certain angles always seems to bounce off unless I use the heavy overhead swing.
Yeah, like Tawa said, the hardness of certain parts is higher. Having a good enough sharpness will allow you to pierce it, but you already knew that. For the Kut-Ku, try aiming for the head or I think the belly.
You sound like you could benefit from using a Sword and Shield. It'll make it easy to counter the fireballs since you're both more mobile and can simply block them if you want, it'll be easier to use the potions since you don't have to sheath, its smaller size makes it easier to hit softer areas to avoid your attacks bouncing. It's a really good weapon in a lot of areas in general, and it's easier to learn than most due to its simplicity.
Also, I'm sorry about talking about weapons so much, but if you haven't yet, in the one-star caravan quests there should be a bunch of quests marked "Training" where you can learn the techniques of the weapons and practice on a Great Jaggi. Even if you keep using the longsword, there are still techniques that you might not know about, like the spirit combo and fade slash, with the former being a crucial part of using the longsword since it basically makes you more powerful the more you attack and is impossible to deflect, which you seem to have already noticed. That's aside from it being a very powerful combo in general.