I had a similar issue with my quad, I have 2 plug in dvbs2 tuners though.
I swapped the tuners round in the slots to see if the fault followed the tuner or the slot. After I did that I tested both tuners and they both started working
I can only assume that the card wasn't sitting in the slot quite right even though it looked like it was. I've had no fault since.
The cards are only secured in the slot with the screw in the back plate so I guess they could pull out. If my tuners ever do it again I'm going to get a bit of sticky foam to put on the underside of the lid of the box to help secure the tuners