Maintaining and improving quality in an established and existing line is just as hard – if not harder – than increasing quality on a line from very poor origins. It takes very little real work to take a line from poor quality to acceptable quality (to a large degree, just feeding, raising, and housing them in better conditions for a few generations will make them much better, nevermind any selection you do). But moving a rat from being acceptable quality to very good quality is harder, and actually maintaining a top quality line is very difficult.
Rat breeding is about spinning lots of plates in the air at once, not about checking things off a list. You can’t breed to get one aspect right, tick it off as “done”, then move on to something else. You need to constantly be spinning each plate enough to ensure that it’s not going to fall down, carefully watching to ensure none of the other plates are wobbling too much.