Sometimes it's not the seeds, but it's you or the media you are germinating in.
About 4 weeks ago I plants 20 new varieties.
4 popped up in the usual 1-2 weeks.
After that, nothing.
4 weeks later I checked them, 7 were rotted and bad seeds.
3 had germinated but didn't make it to the surface as they were watered in too deep (planted fine but they do get buried more if you pour water on them roughly)
The other 6, the seeds seemed too dry. I re-soaked them and in 3 days, 3 of that remaining 6 have popped out already.
Point is, it's not always the seed. Sometimes it's temperature, but also some seeds end up too deep, others get kept too wet or too dry. And that's using coir (and you would get similar results with peat or jiffies), when you are using seed raising or potting mix*, then results would be worse as you are dealing with fungus, bacteria and other nasties prevalent in moist soil you don't get with dried products like coir or jiffies.
* I've had a disastrous start to the season after potting up seedlings into potting mix. IE I germinate fine in coir, but then transplant into some potting mix, it doesn't grow or just gets root rot or stem shrivel (damping off) after transplanting due to fungus or bacteria in the potting mix. If you were trying to germinate in potting mix it might be the crap in the potting mix that is causing the seed not to germinate. Or it could be a mix that is very woody/chippy so the seed dried out as it's surrounded by that and air, not a moist close contact media.
From now on every bag of potting mix I buy gets places in a 40L tub and dried and baked in the sun before I even use it. I'm never letting that stuff get near a seed that needs germinating.