It looks to me as though there was a design fault with the actuators allowing corrosion to occur, causing a main gear failure. This seems to be what happened with the first 2 identical accidents (and grounded a few Flybe aircraft too from what I hear).
Then in haste to change all the actuators, a maintenance error has crept in somewhere and allowed an O-ring to be mis-fitted meaning total non-deployment of RH main landing gear in the 3rd accident.
Of course this is pure speculation on my behalf.