When MDing, if a section has an excerpt of the music that is quite difficult and may require 10-15 minutes of work in order to build understanding of that section, what should the Music Director do so that other sections don't lose interest in the rehearsal and miss out on valuable practice time?
he can skip the difficult passage and look at it next time
play the passage without the whole section or ask the overtaxed players to take a rest during these bars
choose another piece and ask the whole orchestra to practice the difficult piece at home
practice in different sections and send the group with a deputy bandmaster or a section leader (or any other skilled bandmember) to practice in another room.
The most important point is not to blame the members having difficulties and not to demotivate them or the others. E.g. when the bass or tenor section has a problem the chief can choose a piece where the sopranos have their difficulties too.