Complex sentence: To slow the descent, first the drogue parachute was deployed, then the main one; the main parachute did not unfold due to a faulty pressure sensor which had not been detected during manufacture.
Split into simpler sentences: To slow the descent, first the drogue parachute was deployed, followed by the main one. The main parachute did not unfold due to a faulty pressure sensor which had not been detected during manufacture.