There has to be enough sunlight. Sadly, it's the one thing that you can't easily change in a garden.
Talking about herb/flower seeds not germinating... some need stratification. That means they need a cold period (like winter) and then to "wake up" when it gets warm. I tried to grow some New England Asters, and they never sprouted. The packet even said, "Pre stratified." I'm going to try again (I bought 2 packets by mistake) and stratify them in the fridge for a couple of months. I'll have to look up how to do it again. 
I did buy 10 plants and they did VERY well. Once the critters stopped gnawing on them, anyway. Citrus peels seemed to have worked for that.