Have you been target feeding them? Anything picking on one? My pair don't always come out at the same time, and a hermit crab used to sometimes try to hide in the cranny where one tube is--making that worm stay in, even though the other one came out.
no target feeding. Dropping some stuff live stuff that's better than live DT's, and some dead plankton in the tank daily. I haven't noticed anything picking on it.
Have you noticed any peices of the "feather" in the tank? It may have also popped its head and is re generating another one. They wont come out while they are doing this.