TLDR: It was not the EDL [edited to add: entry descent and landing] system, it was the fabrication and integration (putting together) of certain actuators that are used in low temperatures at Mars. People at a contractor, and at JPL, were putting in double shifts and they came within a very close margin of getting it together.
Additionally, there were flight software/avionics issues.
But if you're set to miss the launch window by even a week, you have to wait for the next one 26 months later.
TLDR: It was not the EDL [edited to add: entry descent and landing] system, it was the fabrication and integration (putting together) of certain actuators that are used in low temperatures at Mars. People at a contractor, and at JPL, were putting in double shifts and they came within a very close margin of getting it together.
Additionally, there were flight software/avionics issues.
But if you're set to miss the launch window by even a week, you have to wait for the next one 26 months later.