The D7/D8 will not automatically switch input from analog in to digital in if you insert the 7 pin cable after first pressing record.
So the proper sequence would be to insert the 7-pin cable first and then put the D8 into record mode. The other thing to watch out for is if you have an "active" 7-pin cable that has the analog/digital switch on the front. If you have this type of cable, be sure to have the switch placed to "digital".
If neither of these work, it could be that your 7-pin cable isn't functioning correctly. See if you can test the cable with a friend's D7/D8 to be sure it is working. If it turns out your 7-pin cable is fried, I've got a couple for sale in the Yard Sale.