Evaporation or excessive?
First step, measure water loss with a measuring tape.
Make a small line on skimmer flange at water level.
Measure water loss after 24 hours without rain or backwashing.
If loss is excessive, measure water loss with pump running for 24 hours.
Then measure water loss with pump turned off for 24 hours.
This will isolate the possibilities making the troubleshoot easier.
If you loose water faster with pump running, could be a broken pipe underground or water leaking out by the backwash pipe.
If the water loss is the same with pump running or off, then problem might be a hole in the liner.