GTween animation not always animating

I’m using a GTween to move a character across the screen, so it’s only tweening the x value. I want the character to ‘jump’ when it’s tapped, so I made another GTween object to handle that animation. I’ve set it up like so:

birdTween = new GTween( bird_mc, birdJumpTime );
birdTween.ease = Quadratic.easeOut;
birdTween.reflect = true;
birdTween.repeatCount = 2;

And when the character is tapped, I call the following code:

birdTween.setValue( "y", bird_mc.y - birdBounceHeight );

Which works great… when it works at all. Sometimes the animation doesn’t play at all. It’s not an issue with the mouse events, I’ve checked those and they fire consistently. The code is being executed, it’s just not doing anything, and there doesn’t seem to be any pattern as to when it does and does not work.

Any ideas?