Have you tried rooting hormone? It’s a powder, dip the cut end in it and plant.
Yes - I don’t think that’s the problem with the Opo, though, as these are well established plants. On this last one, the vine was over 10 ft. long and growing fast and flowering well, when the problem hit.
I also have a gallon of liquid rooting hormone (some nutrients included too).
https://www.amazon.com/Green-Light-Organic-Stimulator-Solution/dp/B000BZ8HHW