present continuous, present perfect continuous

[Lenka]

I'm convinced she wasn't paying attention at the time.
= She couldn't have been paying/be paying attention at the time.

Why is it impossible (and not right) to use "be paying" in the sentence?

[Lenka]

Oh... I am sorry, but I don't know how to delete the thread.
I'd just like to tell you that you don't have to answer my question, as I have just realized I do know why the form "have been" was used... At least I believe it's the reason I think.