There is a bug in the current shipping version (fixed in beta update 5.6B2 available from our Web Site) that causes the unit not to initially clock to ADAT.

To see if this is the problem, switch the clock source to something else, then switch it back again and see if ADAT shows LOCKED.

Otherwise, make sure that your ADAT connectors are not too bulky – the connectors are quite close and bulky plugs may not be able to push in properly.