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I was give an O Macrocentra fruit a bit over a month ago. I immediately harvested and cleaned up the seeds and then put them in a takeaway container to germinate. I sowed 5 other types of seeds at the same time including another straight from fruit (H tetracantha) and they have all germinated now.

Some googling and I found this talking about how the seeds need to "ripen" after harvesting. I've never heard of that before. I know some seeds won't germinate unless they experience low temperatures first, I know some won't germinate in strong sunlight etc, but I've never heard that one.

I also have gibberellic acid on hand so I could use that too.

Suggestions?

The seeds need to be scarified - i.e,. nick or scratch the hard outer seed coat to let moisture penetrate. Good video on techniques for this here: https://www.google.com/url?sa=t&rct=j&q ... ifL89H5-la

Soaking seeds in water at room temperature for an hour or two (or a day or two) before sowing is often meant to help with opuntia seeds too.
If you have lots, you can try different approaches to try to find what works best.