Our league requires a bowler to use the book (or entering) average for a specific number of games. After a bowler has completed bowling the required number of games, the average on the standings no longer show BK, yet the average that is shown is still the book and not their current average.
In this situation, most likely you have enabled the pin-drop rule and indicated that the maximum number of pins that a bowler can drop is ZERO.
There are two rules that are independent of each other.
"A bowler's average may not go below the following average" rule is used to set a minimum average that the bowler's average can not go below. So if you do not want any bowler to drop below 140, this rule must be set to 140.
"A bowler's average may not go below the Book/Entering Average more than the following number of pins" rule is used to determine the maximum number of pins that a bowler can drop their average from their book average. So if the league has a bylaw that states a bowler can not drop more than 10 pins than their book average, then this rule must be set to 10.
A common error is when a league uses the first rule, but not the second rule, to which the league secretary sets this value to Zero, thinking that there is no drop, when it actually means that bowlers can not drop more than Zero pins from their book. When not using this rule, it's best to set this value to 99.