I haven't found that oyster shell has much to do with feather re-growth after a molt. It's used to provide calcium and that's all it is - calcium. The biggest help during molt is protein. I feed mine All Flock or a flock maintenance rather than layer food. It's usually higher in protein than the layer feed. They have oyster shell on the side. That way I only have to buy one food for chicks, roos, and layers, yet the girls get the extra calcium they need.
They say all things are done in a season. Well, like yours, mine forgot to read the book that talks about which season is molting season and they'll molt any old time they feel like it. Protein is your friend during this time but really nothing you can do will "make" feathers grow in faster - they'll come in when they are ready. <sigh> Silly chickens.