Your guess of the problem with your seeds could be wrong. There are some tricky parts of growing cactus seed. Some species need wet/dry regimens, some need cold/hot, and some need combinations of both. If you can grow hard to germinate native plants from seed, you are ready for exotics like cactus. I say this because I also assumed when I started that the problem was the seed. The day I threw out a pot with about 20 hard to germinate Blossfeldia liliputiana, I learned my lesson. Some seeds germinate after 2 years, or when conditions allow. They have their own schedule.

Good luck on your quest.

I generally spend a few hours doing searches on each seed variety, in order to find the best method to germinate.
I experiment a little as well, splitting the seeds into groups, trying smoke, hormones (rooting and Gibberellic Acid)scarification.
I have only recently read about wet/dry regimens.
Do you know of any site that would have such germination information specific to the cacti variety? The seed house recommendations are often generalist.

I generally spend a few hours doing searches on each seed variety, in order to find the best method to germinate.
I experiment a little as well, splitting the seeds into groups, trying smoke, hormones (rooting and Gibberellic Acid)scarification.
I have only recently read about wet/dry regimens.
Do you know of any site that would have such germination information specific to the cacti variety? The seed house recommendations are often generalist.

I've had cactus seeds that have taken over 6 months to germinate.

I feel your pain! I recently got some Opuntia violacea seeds and I am having the darndest time getting them to germinate. I've tried soaking. The wet paper towel in a baggie method and letting them dry out and re-soaking them to no avail. I had a bit of luck getting some unknown opuntia seeds to germinate by accident by soaking and drying them out many times, then the wet paper towel/baggie method and they finally sprouted. I have read about scoring the seeds first to get them to sprout, but I'm lost on how to do that without damaging the seed. Anyone have any advice on this?
Also... when I had the violacea seeds in the baggie with the wet paper towel, I noticed an ammonia smell that got stronger as the days went by. Is this normal?

The ammonia doesn't sound good, maybe wash the seed by soaking it in a 5% bleach to water solution for 5 minutes, if they haven't already sprouted, then maybe sow them and hope for the best.
I have tried 2 varieties of Opuntia recently, one lot did ok, the other I'm still waiting on.
Scarification is a simple process, and generally it makes a significant positive difference on any hard coated seed.
For both Opuntia's I used a fine grade of sand paper, nail file would do and filed away one edge of the side of the seed. I think you just want to remove enough of the seed coat to allow moisture to penetrate, sort of mimicking digestion by an animal. So I took maybe a 10th of one edge away. Check it with a magnifying glass, you just barely want to see the inside seed. You could gently wash a few of the sown seeds free, if no signs of germination, try scarifying them