There are 3 belays in Brawl.
The first belay is a tether grab that works when the ice climbers are synched together. It has a very long grab radius, and Nana will teleport from wherever she is to grab the ledge so long as both Ice Climbers are even with or below the ledge. They can be far away from each other, and Nana will still teleport to Popo so long as they are in synch.
The second belay is just a normal recovery that works when the Ice Climbers are synched but above the ledge. Nana will teleport from wherever she is to jump above Popo, then they will both get a boost in height and desynch.
The third belay happens when the Ice Climbers aren't in synch. Either Nana is dead, too far away, or she was separated from Popo and didn't resynch in the air when they got close to each other. They could be right next to each other and still be desynched, depending on how your opponent hit you and what you did (air dodges/attacks) since then. In this case, the Ice Climbers will try to do the belay motion, but nothing will happen because the game thinks they are still separated.