I have run into this situation two or three times this year.

As the trail, there is always an opportunity for a player to shoot a three right in front of you, and not allow a good line of vision to see if her feet were behind the line. If a player is to my side, I will try to move down, but last night she dribbled right in front of me and did not allow an opportunity for me to re-position before she shot it.

Am I doing something wrong?

I have had this happen many times. I went to a camp last summer and learned one thing that helped me a lot in cases like this. As trail, move in the opposite direction that the player is moving. For instance if they dribble from the middle of the court toward the sideline, you move toward the middle. If they move the other way you move down a bit. I used to get burned on 'toe on the line' calls that I counted for 3. So far I haven't had one this season that I know of.

Postioning
 

Also, anticpate where the ball is going to be thrown. That way you can make your movements before the action or player gets in front of you and blocks your lane of vision. Something I am working on is this: Anytime the ball goes below free throw line extended try and work at or a step below the 28 foot line in trail. You'll be amazed at the amount of angles you see and your lane of vision really gets better as well. Plus you cut down the chances of getting "stacked" or straight lined on this 2 or 3 point jump shot your talking about here.