some strains just take longer. and if they're old seeds (2-3 years), they tend to be slower germinating.
I usually see some germination on all strains within 7-10 days, and don't worry until after 14 days. Then, I set it aside, and re-seed or start a 2nd batch. I'll give the 1st batch about 30 days before tossing them out.
I've had germination rates as low as 5%, and over 2-3 weeks before, even with seed from reliable sources. Sometimes it just happens. I'm struggling with YTB also - really slow. They are seeds are from my own last-year's crop, and I had 80%+ germination in 10 days on a 'viability' test last fall. Now, they are just difficult.