It's been a while since they cracked intentional interworld travel, and longer still since Bellona Elric fell between worlds in an array gone wrong. Unfortunately, 'cracked intentional interworld travel' implies something more thorough than what they actually managed, which was sending Bellona's sister to her. It required their linked Gates as a mediator, and now that El and Bellona are in the same world, it seems they can't come back. 

And, annoyingly to Ling, she can't directly use the array, to go out or pull things in. Which means there's a massive opportunity for improving her advantage in Xing's endless power struggles that she's currently missing out on.

(Also she misses her friends, though she won't acknowledge that out loud, and worries about them - she hasn't heard anything since sending El off, though she has enough faith in their array to be sure El has joined her sister.)

The beauty of alkahestry and alchemy alike is that it's possible to modify arrays, though. To improve on them. Which is exactly what Ling is doing now.