I'm wondering if the pushrod has dropped, or moved somehow and is no longer engaging the piece in the center of the clutch. It shouldn't but it's about all I can come up with, or the O ring getting shredded and gumming the whole thing up as has been mentioned
I take it that all you replaced was the rear O ring, did you pull out the whole pluger assembly including the thrust bearing or just the rear section leaving the front piece in place?
First I'd check the O ring is Ok and it's all mating together as it should, if that is all good then, sorry to say, but, I think the box will need to come out to investigate furhter.
John