The most likely reason for this is that the ribbon cable is not seated correctly.
Please note that the connectors have locking tabs (indicated below with a blue arrow) that need to be pulled out for removal and pushed back in to lock the ribbon cable in place. Please also note the orientation of the cable (which side the blue bar is facing) shown below with red arrows.
When inserting the ribbon lightly hold your thumb on top of the connector. You should feel some resistance as the ribbon slides in.
If it seems to float freely into the Fenrir's connector you need to re-insert.
Sometimes its easiest to let the ribbon ride along the right or left side of the connector as you slide it in.
Once you have it connected it will stay connected as the tab is locking and you won't need to do this again.