Optical input is not working or has intermittent problem
Some entry level consumer devices have optical outputs with high jitter and that can affect the locking of optical signal with some model of NuPrime DAC. If this is determined to be the problem, you can use an optical to analog converter such as "Musou Digital Optical Coax to Analog RCA Audio Converter Adapter" (available from Amazon.com) to convert the noisy optical signal to the DAC's analog RCA inputs.