Since the SAMD21 is a 3.3V ONLY processor, if a voltage > 3.3V was ever applied to the analog input, you may have damaged the ADC.
This thread shows how to get 16bit resolution using oversampling and averaging: Addressing SAM D21 ADC
More on getting the most out of the SAMD21 ADC: Reading analog values with the SAMD21's ADC - Stargirl (Thea) Flowers