Most of the time it's the seeds. If there older they may take longer to germ and some could be genetics. I've always had good luck using spagnum moss in a plastic dixie cup under flouresants. The moss holds the moisture. I just germinated a large batch putting about 15 seeds per cup. (they were free from some killer weed a friend had) Even I was amazed that almost everyone sprouted within 2 days. Then I take out the weak ones and keep the healthiest ones until I'm down to 1 per cup. When there about 5' tall I transplant them. Been doing it this way for 10 years plus. Every once in a while I do get seeds that take a while or don't germ at all. Luck of the draw I guess.