Basically, unstable and questionable genetics.
In other words, poor breeding practices, whereby Seed Stockers allegedly outsource their breeders and just market a product without direct involvement.
Next time, try some "Sweet Seeds" seeds, you will be amazed with their quality genetics and reasonable prices.
After growiong for 35+ years, they have become my "go to" seed bank and I recommend them to all growers, new or veterans, as top results are so easily achieved.
Potency, yield, stability, 99.9% germination and ease of cultivation are all reasons why this seedbank are the quiet achievers of the cannabis market.
Hope this helps, Organoman.