I'm not certain what I say is correct(I haven't read Proclus), but it might be of some help to you.
Proclus seems to be saying either all things are uncaused, there is a circular chain of causation, or there is an infinite chain backwards. He rejects the first by pointing out causes do exist, and also notes it would make scientific knowledge impossible(and we certainly do have scientific knowledge, so this can't be the case).
He rejects the second, by pointing out what produces is superior to what is produced, but if there were a circle of causes, then what produces would be both more and less than what it produces, and what is produces would be both more and less than what it is produced by. This cannot be, so we must reject a circle of causes as well.
He rejects the third by pointing out it also makes knowledge impossible. It might be argued here that it would only make certain knowledge impossible. For example, if we have a chain of causes A->B->C->D->E, then we might say "We know the causes of E, D, therefore we know E, even though we do not know D because C is unknowable". Proclus seems to reject such an argument. He seems to be saying "You say you know E because you know the cause D, but I ask you for anything about that cause, and you must answer, if you have knowledge, about the causes of D, C. At least, C cannot be excluded from the answer. But you say you cannot know C, and if you cannot know C, how can you know D, when D depends on C for explanation? You cannot."
It would be like me saying "I know that the power button on my phone turns it on and off" without knowing anything about electricity or circuitry or phone design etc. I do not know that the power button is what turns it on unless I know the cause of its turning on, and while that cause is the power button, if I say I know nothing of the power button, I certainly cannot say I know it is responsible for turning my phone on.
Now that Proclus has rejected all these alternatives, he can reaffirm what he has earlier built the basis for: That all things proceed from a single first cause.