I had that happening earlier in teh season when I didnt have a full screen of clients in the window, once I got he window filled up, the blank at the bottom line disappeared.

I know this doesnt fix your issue, but its not just you thats seen this.

If you keep your EFcenter arranged by Status Date (click on the header at the top of that column) it keeps the active returns up at the top of the list, and the ones you havent touched recently at the bottom.