Flutes will need retuning or other bore work when the Ji plaster inside is disturbed through external cracking or internal moisture. You can see the fissures.
I use tradiitonal Ji paste mixture or modern western plaster depending upon the age and importance of the flute. The traditional work takes much longer where as the Modern plaster is quicker to work with and has proven to hold up just as well. Traditional urushi lacquer is used on the final finish.
There are many kinds of situations that may require bore work, it can be minimal or extensive. Here’s a Seikado shakuhachi before.