Hi Mark,
In addition to Randall's advice, check the raw rain reading in the data editor/viewer - it should smoothly increase throughout the day (assuming it's raining). If it drops to 0 then goes back up again that'll be the problem - just edit the 0 up to the previous value to correct it (reboot WeatherCat after the edit). These zero's shouldn't come through from the console in real time but they sometimes appear in corrupt data logger records.