If the center rail doesn't seem to fit into the frame end, make sure that the rails are properly oriented. Also, check to make sure the set screws on the longer side of the frame end are sufficiently loosened as they can block the rest of the center rail from going all the way down.
The image below shows how the center rails should look when you're sliding them into the frame ends:
The thinner section above the cutout should be on top and the thicker part should be on the bottom.
For troubleshooting purposes, don't worry if it looks like the midpoint mark doesn't appear where it should; the most important thing is that the orientation matches what's shown here.
If your center rails are already oriented this way and still won't attach properly, please contact us so we can determine if it's warped or defective in a way that would block the center rail.