If I say:
It could be interesting
= "the thing had a chance to be interesting" -- either was or wasn't interesting.
It could have been interesting
= "the thing could be interesting, but actually it wasn't."
And what about:
You could do it
= "I don't understand why you didn't do it."
"You could have done it"
= I don't understand why you didn't do it.
Which one is correct?