grammar - Difference between "couldn't" and "wasn't able to"

I don't quite understand a difference between "couldn't" and
"wasn't able to". Someone suggested that we use "wasn't able to" when we talk about one action in the past and "couldn't" when repeated actions in the past. Is that the case?

For example,

• I wasn't able to sleep last night



• I wasn't able to fall asleep last night
  




• I couldn't sleep last night



• I couldn't fall asleep last night

Which ones are right ?