Hopefully any difficulties with the PA system (and the band!) will have been dealt with during the sound-check, although minor adjustments during the performance are often necessary.
Where performances involve cued events these should have been checked and rehearsed during the sound-check.
The sound-engineer(s) will usually use the beginning of the performance to check channels and levels, and will try to introduce any changes that may be needed as inconspicuously as possible. Many minor oversights are possible (a channel may have been muted accidentally, or a performer may have forgotten to switch something on or plug something in). In large-scale multi-performer productions the possibilities are multiplied: wires and microphones can get mixed, and finding what has caused a problem is not always straightforward.
Common difficulties for sound-engineers during performance are cause by musicians changing instrument levels (usually increasing them) between sound-check and performance. This may have several unwanted effects, including:
It is important, therefore, that musicians DO NOT ALTER INSTRUMENT VOLUME SETTINGS AFTER THE SOUND-CHECK.
For theses and similar issues the aim is to get it fixed quickly and unobtrusively, and with larger ensembles there may be more than one issue to address, and/or more than one place to look for a solution. Generally a sound-engineer will find out why he can't hear the keyboard much more quickly if he isn't interrupted by someone telling him they can't hear the keyboard, so if he looks busy, leave him alone!
If performers need adjustments to their monitor settings, it is generally better to tell the monitor or main engineer during a break in the performance, rather than rely on gestures (which can easily be overlooked or misinterpreted).