Do you mean that the up/down/left/right keys don't work at certain times?
Those buttons are actually one button, and trigger the corresponding key press (left, rigth, etc) by determining your finger position using the touchpad.
And because of this the touchpad doesn't work in some cases because you have both your fingers touching the touchpad. (it only likes one finger at the time)
I don't know if this is the case with you, but its a though at least