To put it in baseball terms, Palmer can't put the ball where he wants. His control is off. Its either mechanics or his elbow.
You watch other top-level QBs throw, they throw precisely and can pick on which side of the receiver they throw, high, low, etc etc. Probably to about a 6 inch square window. If given adequate time, they can find the open guy and put the ball where he can catch it and the DB can't.
But it seems that Palmer often makes a throw which makes the catch difficult.
Now, he doesn't get any favors from his receivers. NFL receivers should routinely make the difficult catch. Caldwell especially, Coles, and even sometimes Ochocinco don't seem to do that.