Usually happens if the incoming sample rate is not being locked on or inappropriate for the connection. Check exactly what you are sending, both sample rate (44, 96, etc) and bit (16, 24, 32).
One of two things: You are sending it a signal it’s not able to receive on that input, or it’s not locking onto it correctly.
Keep in mind that S/PDIF standard I believe doesn’t do 192/24 on coaxial, but many DAC makers have extended it. Check your iFi’s specs.