Student or Learner
I have searched the usage of the negative questions of "could" to refer to the past abilities but I could not find anything regarding that.
Here is my understanding: we can use "could" in negative questions to refer to both specific one occasion and general)
- Why couldn't you carry that table yesterday? (=specific occasion)
- Couldn't you speak until the age of five? (=genera ability)
I don't think the distinction between "one specific occasion" and "in general" is relevant to explaining the usage of "could" in negative questions.