TOSlink ADAT Cables can be relatively fragile, as they are made of glass within the cable. They pass light which carries a digital audio signal. Each end of the cable typically comes with a small plastic cover, which looks like a small bubble, which needs to be removed in order to fit the cable properly into the port.
When you remove this cover you will see a very tiny flat glass lens. This lens meets up inside the ADAT Port with another small lens creating a tight fit so that light can pass. You will see a red light emanating from the ADAT Out Ports on your unit, or from the cable itself when it is sending a signal.
Once the plastic cover is removed the cable should seat in the port easily. Forcing the cable unnecessarily can damage the ADAT port doors, which are important to keep the cable secure.
Also be mindful of proper vertical alignment of the connector, as you will see the ADAT Port and Connector have small bevels so that they only fit one way.
Additionally, any small bends or kinks in the cable can potentially break the glass inside the cable and cause it to not pass the audio stream properly.
If you are having any issues with your audio stream you should first check that your units are properly Synced. This means that one unit is receiving clock data from the other unit. If both units are set to "Internal" you can get digital jitter or crackly audio.