Lots of things can lead to more watery insipid flavor in mandarins. For some varieties, it takes several years to develop decent flavor, and many more years for that flavor to slowly improve.
Dancy is supposedly supposed to be the gold standard for mandarin flavor (or so I have read), but I personally do not care for that type of flavor myself.
(I am wondering, are you sure it's not just personal preference and mandarin variety?)
Sometimes too much watering while the fruits are developing can lead to more washed out flavor.
Also sometimes climate can make a difference with some citrus.