Sometimes you go to extend and there are no options that you want - as in the only option is 24 hours or a longer or shorter option than you desire.
This is because whatever rates are available are the ones that the location has made available.
Most locations do not have small amounts of time under an hour, and some locations only have flat rates available especially on evenings and weekends.
Some locations have maximums where you can't extend once you have paid the maximum for the day.
For more information on maximums see more about rate maximums here.