Anonymous“Would you say there is no difference ... ”
I can't possibly think of all the sentences that might be possible with those words, so no, I wouldn't say there is
no difference. There may be a difference I'm not aware of. Maybe it's just that I can't think of a counterexample at the moment. In general, though, I don't sense a great deal of difference, if any, in that specific context.
I looked all over for my wallet, but I [couldn't / wasn't able to] find it.
Even at age 18, Karen's brother still [couldn't / wasn't able to] read.
Susan and Sally tried several times to lift the refrigerator, but they just [couldn't / weren't able to] do it.
Walter [couldn't / wasn't able to] speak Russian five years ago, but he's quite fluent now.
If it's of any interest to you, I tend to use couldn't in preference to wasn't/weren't able to in all those sentences above.
CJ