double barline not appearing

Why does the cello part not have a double barline in the 2nd system?
String Quartet (2).zip (616 KB)

Fixed it by applying a double barline + alt, but still want this to be a learning experience.

You have a local 3/4 time signature at that position in the cello: delete it, and you’ll get the global time signature and the barline “showing through” there.