If a zone is slowly drying out during the watering season, I see 3 possible reasons - the weather data is not local enough, and you have more ET than predicted from the weather data, or your zone parameters (crop coefficient, shade, etc) are not close for one or more zones, or your precip rate is set higher than the actual in a zone, so you are not watering long enough.
Here is a simplistic approach to start tuning your zones.
If all of your zones are affected, then maybe it is weather data. Look for a closer weather station. If none are available, then tweak sun/shade or crop coefficient.
If it is worse in hot dry weather, I would look at zone parameters.
If you are always dry in the zone, then adjust the nozzle precip rate for the zone.
As @Linn suggests, if you do the catch cup test, and dial in the precip rate, then the next lever would be crop coefficient.