You're probably either going too slow, too fast, or you're holding the Triangle button down prior to launching your enemy.
Anyway, I'm not sure as to what weapon you're using, but feel free to look at the following thread for general tips:
Performing the Izuna Drop can be very difficult at first, but it's actually quite easy. I will tell you exactly how I figured it out.
First, the Izuna Drop is Sq, Tr, Sq, Sq, Sq, Tr (with the Dragon Sword, other weapons)
Second, you could perform it with the Lunar Staff, which is even easier - Sq, Tr, Sq, Sq, T
1) Keep the left of your thumb on Square and keep the right of your thumb on Triangle
2) While keeping your finger off the Analog Stick, quickly tap Sq with the left of your thumb, followed by Tr with the right of your thumb
3) With your enemy in the air, quickly press Sq, Sq, Sq, then HOLD Triangle
I found it to be more successful if I conclude the combo by holding Triangle, or else Ryu just does a final slash to pounce the enemy on the ground.