Probably a bad or oxidized connector inside the unit.
Screwdriver time.
If you are brave, open it up, documenting your actions with a camera so you can put it back together properly.
Remove the ribbon connectors and give them a squirt of Deoxit contact cleaner, then reassemble, being careful not to reverse or offset or bend any contact pins on the headers.
Otherwise it is a lack of continuity somewhere in the circuit.
Check out youtube to see how other people have handled it.
If you are not able to DIY this type of repair a technician should get it working for you for around $100- $150.
Gary