Is the layer locked?
No, it is not. :/
I don't know how exactly you're setting up your layout, but I wonder why your "linked" frames aren't surrounded by brackets in the Links panel.
Maybe the reason why it's not working is the method you're using to place the parent and children frames. Look at these screenshots to see how different operations bring different outputs:
1. This is the initial setting (notice that the Links panel show 7 instances of the image and 4 instances of the original frame):
2. This second screenshot was taken after I changed the size, shape and content of the parent frame, but before I updated the other frames. Now the Links panel has 1 instance of the new image, 6 instances of the old image and 4 instances of the linked frames. Notice the yellow triangles (or lack thereof) in the six copies of the original.
3. Finally, this is what happened after I update the frames. As you can see, the four linked frames (either in the page or in the master) inherited the size, shape and content of the original frame, which is located in the page (not the master). Actually, this should work even if the original frame was located in a different document!
As you can see, the only situation in which the frames doesn't update is when you copy and paste the original frame, because doing this doesn't create a link between these copies and the original, it only duplicates de original frame; these copies are neither parents nor children of anything.
So, check your workflow and see whether you're using the place and link command in the correct way: select the original frame, call Place and Link, then use only the conveyor to place copies whenever you want.