An excellent way to perform a retrospective is to ask the Five Whys: Ask why five times to understand the root cause of a problem.
Here is an example:
Problem: The server is slow.
Why #1: Because it is running out of memory.
Why #2: Because the code has a memory leak.
Why #3: Because it was not tested properly.
Why #4: Because we do not have good automated tests.
Why #5: Because we have not received training in automated testing.